Species Recorded | Taxon | Scientific Name | Common Name | Records | Latest |
| alder fly, lacewing or snake fly | Osmylus fulvicephalus | a stream lacewing | 5 | 2014 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Odynerus (Odynerus) melanocephalus | Black Headed Mason Wasp | 3 | 2000 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Oedemopsis scabricula | an ichneumon wasp | 2 | 2003 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Oiorhinus pallipalpis | an ichneumon wasp | 3 | 2002 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Opheltes glaucopterus | an insect - hymenopteran | 1 | 2018 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Ophion | an Ophion sp. (unidentified) | 1 | 2017 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Ophion luteus | an ichneumon wasp | 10 | 2016 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Ophion minutus | an ichneumon wasp | 1 | 2008 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Ophion obscuratus | an ichneumon wasp | 4 | 1997 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Opius caesus | an insect - hymenopteran | 1 | 2008 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Opius pallipes | a braconid wasp | 1 | 2008 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Orthopelma mediator | an ichneumon wasp | 15 | 2003 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Osmia | a mason bee (unidentified) | 2 | 2013 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Osmia (Chalcosmia) caerulescens | Blue Mason Bee | 12 | 2019 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Osmia (Chalcosmia) leaiana | a solitary bee | 4 | 2002 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Osmia (Helicosmia) aurulenta | Gold-fringed Mason Bee | 3 | 2000 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Osmia (Neosmia) bicolor | Two Coloured Mason Bee | 13 | 2019 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Osmia (Osmia) bicornis | Red Mason Bee | 68 | 2020 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Osmia spinulosa | Spined Mason Bee | 6 | 2020 |
| ant, bee, wasp, sawfly or ichneumon (and others) | Oxybelus uniglumis | Common Spiny Digger Wasp | 3 | 2000 |
| beetle | Ochina ptinoides | Ivy Boring Beetle | 11 | 2018 |
| beetle | Ochthebius (Asiobates) dilatatus | a Beetle | 13 | 2015 |
| beetle | Ochthebius (Asiobates) dilatatus | a small water beetle | 6 | 2002 |
| beetle | Ochthebius (Homalochthebius) minimus | a Beetle | 10 | 2012 |
| beetle | Ochthebius (Homalochthebius) minimus | a small water beetle | 9 | 2016 |
| beetle | Ochthebius (Hymenodes) nanus | a Beetle | 1 | 2000 |
| beetle | Ochthebius (Ochthebius) marinus | a small water beetle | 4 | 2002 |
| beetle | Ochthebius (Ochthebius) viridis | a Beetle | 1 | 2009 |
| beetle | Ochthephilum | an Ochthephilum sp. (unidentified) | 1 | 2016 |
| beetle | Octotemnus glabriculus | a small fungus beetle | 2 | 2018 |
| beetle | Ocypus (Ocypus) olens | Devil's Coach-horse | 81 | 2020 |
| beetle | Ocypus (Pseudocypus) aeneocephalus | a Beetle | 1 | 2015 |
| beetle | Ocypus (Pseudocypus) fortunatarum | a rove beetle | 1 | 1978 |
| beetle | Ocys harpaloides | a ground beetle | 13 | 2018 |
| beetle | Ocys quinquestriatus | a ground beetle | 1 | 2002 |
| beetle | Odeles marginata | a marsh beetle | 2 | 1978 |
| beetle | Oedemera | an Oedemera sp. (unidentified) | 3 | 2017 |
| beetle | Oedemera nobilis | Swollen-thighed Beetle | 575 | 2020 |
| beetle | Oedemera (Oedemera) lurida | a thick-legged flower beetle | 211 | 2019 |
| beetle | Oedemera (Oncomera) femoralis | a thick-legged flower beetle | 34 | 2021 |
| beetle | Olibrus aeneus | a smut beetle | 2 | 2001 |
| beetle | Olibrus affinis | a smut beetle | 4 | 2018 |
| beetle | Olibrus liquidus | a smut beetle | 3 | 2018 |
| beetle | Olisthopus rotundatus | a ground beetle | 2 | 1998 |
| beetle | Olophrum piceum | a rove beetle | 2 | 1993 |
| beetle | Omalium caesum | a rove beetle | 4 | 2017 |
| beetle | Omalium laeviusculum | a rove beetle | 6 | 2018 |
| beetle | Omalium rivulare | a rove beetle | 3 | 2016 |
| beetle | Omalium rugatum | a rove beetle | 2 | 2016 |
| beetle | Omaloplia ruricola | a dung beetle or chafer | 4 | 2017 |
| beetle | Omonadus floralis | an antlike beetle | 8 | 2018 |
| beetle | Omosita colon | a pollen or sap beetle | 11 | 2003 |
| beetle | Omosita discoidea | a pollen or sap beetle | 2 | 1981 |
| beetle | Omphalapion hookerorum | a seed weevil | 5 | 2001 |
| beetle | Ontholestes murinus | a rove beetle | 7 | 2019 |
| beetle | Ontholestes tessellatus | a rove beetle | 3 | 2020 |
| beetle | Onthophagus (Paleonthophagus) coenobita | a dung beetle or chafer | 7 | 2020 |
| beetle | Onthophagus (Paleonthophagus) joannae | a Beetle | 2 | 2000 |
| beetle | Onthophagus (Paleonthophagus) joannae | a dung beetle or chafer | 7 | 2020 |
| beetle | Onthophagus (Paleonthophagus) similis | a dung beetle or chafer | 2 | 2000 |
| beetle | Onthophagus (Paleonthophagus) vacca | a dung beetle or chafer | 4 | 1997 |
| beetle | Onthophagus (Paleonthophagus) verticicornis | a dung beetle or chafer | 2 | 1886 |
| beetle | Oodes helopioides | a ground beetle | 1 | 1983 |
| beetle | Oomorphus concolor | a Beetle | 3 | 2015 |
| beetle | Oomorphus concolor | a leaf beetle | 10 | 2018 |
| beetle | Opatrum sabulosum | a darkling beetle | 1 | 2005 |
| beetle | Ophonus (Metophonus) puncticeps | a ground beetle | 5 | 2015 |
| beetle | Ophonus (Metophonus) rufibarbis | a ground beetle | 15 | 2015 |
| beetle | Ophonus (Ophonus) ardosiacus | a ground beetle | 4 | 2011 |
| beetle | Orchesia micans | a false darkling beetle | 1 | 2017 |
| beetle | Orchesia undulata | a false darkling beetle | 1 | 2010 |
| beetle | Orchestes | an Orchestes species (unidentified) | 1 | 2014 |
| beetle | Orchestes (Orchestes) alni | a weevil | 6 | 2018 |
| beetle | Orchestes (Orchestes) pilosus | a weevil | 2 | 2018 |
| beetle | Orchestes (Orchestes) quercus | a weevil | 3 | 2015 |
| beetle | Orchestes (Orchestes) rusci | a weevil | 7 | 2016 |
| beetle | Orchestes (Orchestes) signifer | a weevil | 9 | 2018 |
| beetle | Orchestes (Salius) fagi | Beech Leaf Miner | 16 | 2018 |
| beetle | Orsodacne cerasi | a leaf beetle | 9 | 2018 |
| beetle | Orthochaetes setiger | a weevil | 4 | 1938 |
| beetle | Orthocis alni | a small fungus beetle | 2 | 2016 |
| beetle | Orthoperus atomus | a minute fungus beetle | 1 | 2003 |
| beetle | Osphya bipunctata | a false darkling beetle | 3 | 2017 |
| beetle | Othius punctulatus | a rove beetle | 10 | 2018 |
| beetle | Othius subuliformis | a rove beetle | 4 | 2015 |
| beetle | Otiorhynchus (Dorymerus) rugifrons | a Strawberry Root Weevil | 2 | 1968 |
| beetle | Otiorhynchus (Dorymerus) singularis | Raspberry Weevil | 17 | 2019 |
| beetle | Otiorhynchus (Dorymerus) sulcatus | Vine Weevil | 70 | 2019 |
| beetle | Otiorhynchus (Otiorhynchus) clavipes | Plum Weevil | 40 | 2017 |
| beetle | Otiorhynchus (Otiorhynchus) ligneus | a weevil | 1 | 2008 |
| beetle | Otiorhynchus (Otiorhynchus) rugosostriatus | a Strawberry Root Weevil | 3 | 2005 |
| beetle | Otiorhynchus (Tournieria) crataegi | a weevil | 1 | 2008 |
| beetle | Oulema | an Oulema sp. (unidentified) | 2 | 2016 |
| beetle | Oulema erichsoni | a leaf beetle | 2 | 2009 |
| beetle | Oulema melanopus (s.l.) | Cereal Leaf Beetle | 63 | 2019 |
| beetle | Oulema melanopus (s.str.) | a Beetle | 7 | 2015 |
| beetle | Oulema obscura | a Beetle | 12 | 2019 |
| beetle | Oulema rufocyanea | a Cereal Leaf Beetle | 3 | 2015 |
| beetle | Oulema rufocyanea/melanopus agg. | Cereal Leaf Beetles agg. | 2 | 2017 |
| beetle | Oulimnius | a Oulimnius riffle beetle (unidentified) | 8 | 2012 |
| beetle | Oulimnius tuberculatus | a riffle beetle | 3 | 1997 |
| beetle | Oxypoda alternans | a rove beetle | 1 | 2020 |
| beetle | Oxyporus rufus | a rove beetle | 1 | 1986 |
| beetle | Oxypselaphus obscurus | a Beetle | 10 | 2015 |
| beetle | Oxystoma cerdo | a seed weevil | 10 | 2018 |
| beetle | Oxystoma pomonae | a seed weevil | 17 | 2015 |
| beetle | Oxystoma subulatum | a seed weevil | 24 | 2010 |
| beetle | Oxytelus laqueatus | a rove beetle | 4 | 2020 |
| bird | Oceanodroma leucorhoa | Leach's Petrel | 25 | 2019 |
| bird | Oenanthe deserti | Desert Wheatear | 5 | 2013 |
| bird | Oenanthe oenanthe | Wheatear | 4629 | 2020 |
| bird | Oenanthe oenanthe subsp. leucorhoa | Greenland Wheatear | 9 | 2019 |
| bird | Oriolus oriolus | Golden Oriole | 14 | 2018 |
| bird | Otis tarda | Great Bustard | 4 | 2010 |
| bird | Oxyura jamaicensis | Ruddy Duck | 601 | 2012 |
| bird | Oxyura leucocephala | White-headed Duck | 6 | 2019 |
| bug | Oncopsis alni | a leafhopper | 2 | 2018 |
| bug | Oncopsis carpini | a leafhopper | 1 | 2004 |
| bug | Oncopsis flavicollis | a leafhopper | 5 | 2018 |
| bug | Oncopsis subangulata | a leafhopper | 1 | 2018 |
| bug | Oncotylus (Oncotylus) viridiflavus | a plantbug or grassbug | 3 | 1999 |
| bug | Opsius stactogalus | a leafhopper | 3 | 2018 |
| bug | Orius | an Orius sp. (unidentified) | 3 | 2018 |
| bug | Orius (Heterorius) laticollis | a flower bug or bloodsucking bug | 1 | 2018 |
| bug | Orius (Heterorius) vicinus | a flower bug or bloodsucking bug | 2 | 2008 |
| bug | Orius (Orius) laevigatus | a flower bug or bloodsucking bug | 18 | 2018 |
| bug | Orius (Orius) niger | a flower bug or bloodsucking bug | 11 | 2013 |
| bug | Orsillus depressus | a ground bug | 1 | 2017 |
| bug | Orthocephalus coriaceus | a plantbug or grassbug | 1 | 2000 |
| bug | Orthocephalus saltator | a plantbug or grassbug | 4 | 1999 |
| bug | Orthonotus rufifrons | a plantbug or grassbug | 2 | 2015 |
| bug | Orthops | an Orthops sp. (unidentified) | 1 | 2008 |
| bug | Orthops (Orthops) basalis | a plantbug or grassbug | 2 | 2017 |
| bug | Orthops (Orthops) campestris | a plantbug or grassbug | 38 | 2019 |
| bug | Orthops (Orthops) kalmii | a plantbug or grassbug | 2 | 2015 |
| bug | Orthotylus | an Orthotylus sp. (unidentified) | 1 | 2008 |
| bug | Orthotylus (Litocoris) ericetorum | a plantbug or grassbug | 7 | 2016 |
| bug | Orthotylus (Melanotrichus) flavosparsus | a plantbug or grassbug | 3 | 2009 |
| bug | Orthotylus (Orthotylus) marginalis | Dark Green Apple Capsid | 4 | 2015 |
| bug | Orthotylus (Orthotylus) ochrotrichus | a plantbug or grassbug | 1 | 1978 |
| bug | Orthotylus (Orthotylus) tenellus | a plantbug or grassbug | 1 | 2020 |
| bug | Orthotylus (Pachylops) adenocarpi | a plantbug or grassbug | 2 | 1999 |
| bug | Orthotylus (Pachylops) virescens | a plantbug or grassbug | 2 | 1995 |
| caddis fly | Odontocerum albicorne | a caddisfly | 3 | 2014 |
| caddis fly | Oecetis lacustris | a caddisfly | 1 | 2012 |
| caddis fly | Oecetis ochracea | a caddisfly | 5 | 2014 |
| crustacean | Orchestia | an Orchestia sp. (unidentified) | 1 | 2012 |
| crustacean | Orchestia gammarellus | a sand hopper | 16 | 2017 |
| crustacean | Orchestia mediterranea | a crustacean | 1 | 1940 |
| crustacean | Ostracoda | a mussel shrimp (unidentified) | 18 | 2013 |
| dragonfly, hawker or damselfly | Odonata | a dragonfly, darter, chaser, hawker or damselfly (unidentified) | 164 | 2020 |
| dragonfly, hawker or damselfly | Orthetrum | an Orthetrum sp. (unidentified) | 1 | 1997 |
| dragonfly, hawker or damselfly | Orthetrum cancellatum | Black-tailed Skimmer | 740 | 2019 |
| dragonfly, hawker or damselfly | Orthetrum coerulescens | Keeled Skimmer | 9 | 2012 |
| fish | Oncorhynchus mykiss | Rainbow Trout | 14 | 2011 |
| fungus | Oidium ericinum | a conidial powdery mildew | 1 | 2013 |
| fungus | Ombrophila violacea | a fungus | 2 | 2008 |
| fungus | Omphalina pyxidata | a basidiomycete fungus | 3 | 1997 |
| fungus | Opegrapha parasitica | a lichen | 5 | 2008 |
| fungus | Opegrapha rupestris | a lichen | 3 | 2018 |
| fungus | Ophiobolus vulgaris | a fungus | 1 | 2002 |
| fungus | Ophiocordyceps gracilis | an ascomycete fungus | 1 | 2005 |
| fungus | Ophiostoma ulmi | Dutch Elm Disease | 1 | 1997 |
| fungus | Orbilia | an Orbilia sp. (unidentified) | 1 | 1999 |
| fungus | Orbilia alnea | an ascomycete fungus | 2 | 1999 |
| fungus | Orbilia curvatispora | an ascomycete fungus | 1 | 1999 |
| fungus | Orbilia cyathea | an ascomycete fungus | 1 | 2015 |
| fungus | Orbilia luteorubella | an ascomycete fungus | 1 | 1999 |
| fungus | Orbilia sarraziniana | an ascomycete fungus | 1 | 2005 |
| fungus | Orbilia xanthostigma | an ascomycete fungus | 9 | 2014 |
| fungus | Ossicaulis lignatilis | a basidiomycete fungus | 4 | 2013 |
| fungus | Otidea | a Otidea lichen or fungus (unidentified) | 1 | 1970 |
| fungus | Otidea alutacea | a fungus | 5 | 2013 |
| fungus | Otidea cochleata | a fungus | 2 | 2006 |
| fungus | Otidea onotica | a fungus | 2 | 2013 |
| fungus | Oudemansiella mucida | Porcelain Fungus | 51 | 2018 |
| fungus | Oxyporus populinus | a basidiomycete fungus | 8 | 2011 |
| grasshopper, cricket or true cricket | Omocestus rufipes | Woodland Grasshopper | 11 | 2019 |
| grasshopper, cricket or true cricket | Omocestus viridulus | Common Green Grasshopper | 105 | 2018 |
| grasshopper, cricket or true cricket | Orthoptera | an Orthoptera sp. (unidentified) | 8 | 2020 |
| harvestman | Odiellus spinosus | a harvestman | 4 | 2018 |
| harvestman | Oligolophus tridens | a harvestman | 9 | 2017 |
| harvestman | Opilio | an Opilio sp. (unidentified) | 2 | 2016 |
| harvestman | Opilio canestrinii | a harvestman (Opiliones) | 11 | 2019 |
| harvestman | Opiliones | a harvestman (unidentified) | 21 | 2018 |
| harvestman | Opilio parietinus | a harvestman | 6 | 2015 |
| harvestman | Opilio saxatilis | a harvestman | 12 | 2018 |
| lichen | Ochrolechia androgyna | a lichen | 2 | 1996 |
| lichen | Ochrolechia parella | a lichen | 7 | 2016 |
| lichen | Opegrapha | an Opegrapha sp. (unidentified) | 3 | 2019 |
| lichen | Opegrapha atra | a lichen | 21 | 2019 |
| lichen | Opegrapha calcarea | a lichen | 8 | 2018 |
| lichen | Opegrapha dolomitica | a lichen | 1 | 2018 |
| lichen | Opegrapha herbarum | a lichen | 3 | 1998 |
| lichen | Opegrapha mougeotii | a lichen | 1 | 2002 |
| lichen | Opegrapha vermicellifera | a lichen | 1 | 1988 |
| lichen | Opegrapha vulgata | a lichen | 6 | 2020 |
| mammal | Oryctolagus cuniculus | Rabbit | 1782 | 2021 |
| mammal - marine | Odobenus rosmarus | Walrus | 1 | 1839 |
| mammal - marine | Orcinus orca | Killer Whale | 3 | 1902 |
| millipede | Ommatoiulus sabulosus | Striped Millipede | 3 | 2019 |
| millipede | Ophiodesmus albonanus | a millipede | 1 | 1987 |
| millipede | Ophyiulus pilosus | a julid snake millipede | 7 | 2012 |
| mollusc | Omphiscola | a Omphiscola pond snail (unidentified) | 6 | 2007 |
| mollusc | Onchidoris muricata | a mollusc | 3 | 1939 |
| mollusc | Opisthobranchia | an Opisthobranchia sp. (unidentified) | 1 | 2015 |
| mollusc | Ostrea edulis | Common Oyster | 2 | 2019 |
| mollusc | Oxychilus | a glass snail (unidentified) | 11 | 2018 |
| mollusc | Oxychilus (Oxychilus) alliarius | Garlic Snail | 53 | 2016 |
| mollusc | Oxychilus (Oxychilus) cellarius | Cellar Snail | 65 | 2018 |
| mollusc | Oxychilus (Oxychilus) draparnaudi | Draparnaud's Snail | 24 | 2015 |
| mollusc | Oxychilus (Oxychilus) navarricus subsp. helveticus | Glossy Glass Snail | 38 | 2016 |
| mollusc | Oxyloma (Oxyloma) elegans | Pfeiffer's Amber Snail | 111 | 2016 |
| moss | Orthodontium lineare | Cape Thread-moss | 11 | 2018 |
| moss | Orthotrichum | an Orthotrichum sp. (unidentified) | 2 | 2013 |
| moss | Orthotrichum affine | Wood Bristle-moss | 82 | 2019 |
| moss | Orthotrichum anomalum | Anomalous Bristle-moss | 24 | 2019 |
| moss | Orthotrichum diaphanum | White-tipped Bristle-moss | 25 | 2019 |
| moss | Orthotrichum lyellii | Lyell's Bristle-moss | 11 | 2019 |
| moss | Orthotrichum pulchellum | Elegant Bristle-moss | 18 | 2019 |
| moss | Orthotrichum stramineum | Straw Bristle-moss | 5 | 2018 |
| moss | Orthotrichum striatum | Shaw's Bristle-moss | 2 | 2018 |
| moss | Orthotrichum tenellum | Slender Bristle-moss | 5 | 2018 |
| moss | Oxyrrhynchium hians | Swartz's Feather-moss | 42 | 2019 |
| moss | Oxystegus sinuosus | Wavy Beard-moss | 24 | 2018 |
| moth or butterfly - butterfly | Ochlodes sylvanus | Large Skipper | 5354 | 2020 |
| moth or butterfly - macro moth | Ochropacha duplaris | Common Lutestring | 128 | 2020 |
| moth or butterfly - macro moth | Ochropleura plecta | Flame Shoulder | 3992 | 2020 |
| moth or butterfly - macro moth | Odezia atrata | Chimney Sweeper | 81 | 2019 |
| moth or butterfly - macro moth | Odontopera bidentata | Scalloped Hazel | 438 | 2020 |
| moth or butterfly - macro moth | Odontosia carmelita | Scarce Prominent | 1 | 1905 |
| moth or butterfly - macro moth | Oligia | a Oligia minor moth (unidentified) | 302 | 2020 |
| moth or butterfly - macro moth | Oligia fasciuncula | Middle-barred Minor | 475 | 2020 |
| moth or butterfly - macro moth | Oligia latruncula | Tawny Marbled Minor | 896 | 2019 |
| moth or butterfly - macro moth | Oligia strigilis | Marbled Minor | 810 | 2020 |
| moth or butterfly - macro moth | Oligia strigilis agg. | Marbled Minor agg. | 640 | 2020 |
| moth or butterfly - macro moth | Oligia versicolor | Rufous Minor | 303 | 2017 |
| moth or butterfly - macro moth | Omphaloscelis lunosa | Lunar Underwing | 1548 | 2020 |
| moth or butterfly - macro moth | Operophtera brumata | Winter Moth | 435 | 2020 |
| moth or butterfly - macro moth | Operophtera fagata | Northern Winter Moth | 6 | 2005 |
| moth or butterfly - macro moth | Opisthograptis luteolata | Brimstone Moth | 4882 | 2020 |
| moth or butterfly - macro moth | Orgyia antiqua | Vapourer | 330 | 2020 |
| moth or butterfly - macro moth | Oria musculosa | Brighton Wainscot | 1 | 1976 |
| moth or butterfly - macro moth | Orthonama vittata | Oblique Carpet | 17 | 2007 |
| moth or butterfly - macro moth | Orthosia cerasi | Common Quaker | 2249 | 2020 |
| moth or butterfly - macro moth | Orthosia cruda | Small Quaker | 655 | 2020 |
| moth or butterfly - macro moth | Orthosia gothica | Hebrew Character | 3235 | 2020 |
| moth or butterfly - macro moth | Orthosia gracilis | Powdered Quaker | 305 | 2020 |
| moth or butterfly - macro moth | Orthosia incerta | Clouded Drab | 1591 | 2020 |
| moth or butterfly - macro moth | Orthosia miniosa | Blossom Underwing | 7 | 2009 |
| moth or butterfly - macro moth | Orthosia opima | Northern Drab | 9 | 2016 |
| moth or butterfly - macro moth | Orthosia populeti | Lead-coloured Drab | 4 | 2012 |
| moth or butterfly - macro moth | Ourapteryx sambucaria | Swallow-tailed Moth | 1024 | 2020 |
| moth or butterfly - micro moth | Ochsenheimeria taurella | a micro-moth | 7 | 2017 |
| moth or butterfly - micro moth | Ochsenheimeria vacculella | a micro-moth | 4 | 2016 |
| moth or butterfly - micro moth | Ocnerostoma friesei | a micro-moth | 2 | 2010 |
| moth or butterfly - micro moth | Oecophoridae | an Oecophoridae sp. (unidentified) | 1 | 2013 |
| moth or butterfly - micro moth | Oegoconia | an Oegoconia sp. (unidentified) | 76 | 2018 |
| moth or butterfly - micro moth | Oegoconia deauratella | a micro-moth | 5 | 2018 |
| moth or butterfly - micro moth | Oegoconia quadripuncta | Four-spotted Obscure | 208 | 2019 |
| moth or butterfly - micro moth | Oidaematophorus lithodactyla | Dusky Plume | 7 | 2014 |
| moth or butterfly - micro moth | Olethreutinae | an Olethreutinae sp. (unidentified) | 1 | 2011 |
| moth or butterfly - micro moth | Olindia schumacherana | a tortrix moth | 14 | 2018 |
| moth or butterfly - micro moth | Oncocera semirubella | a pyralid moth | 4 | 2014 |
| moth or butterfly - micro moth | Opostega salaciella | a micro-moth | 1 | 2017 |
| moth or butterfly - micro moth | Ortholepis betulae | Birch Knot-horn | 3 | 2001 |
| moth or butterfly - micro moth | Orthotaenia undulana | a tortrix moth | 7 | 2011 |
| moth or butterfly - micro moth | Orthotelia sparganella | a micro-moth | 6 | 2013 |
| moth or butterfly - micro moth | Ostrinia nubilalis | European Corn Borer | 69 | 2018 |
| moth or butterfly - micro moth | Oxyptilus parvidactyla | a plume moth | 7 | 2003 |
| plant | Odontites vernus | Red Bartsia | 994 | 2020 |
| plant | Odontites vernus subsp. serotinus | a red bartsia | 1 | 1989 |
| plant | Oenanthe | a water-dropwort (unidentified) | 10 | 2013 |
| plant | Oenanthe aquatica | Fine-leaved Water-dropwort | 27 | 2010 |
| plant | Oenanthe crocata | Hemlock Water-dropwort | 2094 | 2020 |
| plant | Oenanthe fistulosa | Tubular Water-dropwort | 157 | 2016 |
| plant | Oenanthe fluviatilis | River Water-dropwort | 1 | 2002 |
| plant | Oenanthe lachenalii | Parsley Water-dropwort | 50 | 2016 |
| plant | Oenanthe pimpinelloides | Corky-fruited Water-dropwort | 315 | 2020 |
| plant | Oenothera | an evening primrose (unidentified) | 107 | 2017 |
| plant | Oenothera biennis | Common Evening-primrose | 51 | 2018 |
| plant | Oenothera cambrica | Small-flowered Evening-primrose | 37 | 2016 |
| plant | Oenothera glazioviana | Large-flowered Evening-primrose | 191 | 2019 |
| plant | Oenothera glazioviana x biennis = O. x fallax | an evening-primrose | 1 | 2016 |
| plant | Oenothera indecora | a Plant | 1 | 1923 |
| plant | Oenothera laciniata | a Plant | 1 | 1922 |
| plant | Oenothera stricta | Fragrant Evening-primrose | 8 | 2013 |
| plant | Oleaceae | an Oleaceae sp. (unidentified) | 1 | 2017 |
| plant | Olearia | an Olearia sp. (unidentified) | 1 | 2013 |
| plant | Omphalodes verna | Blue-eyed Mary | 4 | 2016 |
| plant | Onobrychis crista-galli | a Plant | 1 | 1937 |
| plant | Onobrychis viciifolia | Sainfoin | 72 | 2017 |
| plant | Ononis | an Ononis sp. (unidentified) | 63 | 2013 |
| plant | Ononis repens | Common Restharrow | 799 | 2019 |
| plant | Ononis spinosa | Spiny Restharrow | 333 | 2019 |
| plant | Onopordum acanthium | Cotton Thistle | 8 | 2007 |
| plant | Ophioglossum vulgatum | Adder's-tongue | 178 | 2019 |
| plant | Ophioglossum vulgatum (sens.lat.) | Adder's-tongue (sens. lat.) | 7 | 2005 |
| plant | Ophrys apifera | a bee orchid | 1 | 2002 |
| plant | Ophrys apifera | Bee Orchid | 553 | 2020 |
| plant | Ophrys apifera var. friburgensis | a bee orchid | 10 | 2007 |
| plant | Ophrys insectifera | Fly Orchid | 72 | 2015 |
| plant | Ophrys insectifera x apifera | a spider-orchid | 2 | 1988 |
| plant | Orchidaceae | an orchid (unidentified) | 77 | 2020 |
| plant | Orchis mascula | Early-purple Orchid | 462 | 2020 |
| plant | Orchis purpurea | Lady Orchid | 2 | 1999 |
| plant | Origanum | an Origanum sp. (unidentified) | 3 | 2020 |
| plant | Origanum vulgare | Wild Marjoram | 1007 | 2019 |
| plant | Ornithogalum angustifolium | Star-of-Bethlehem | 48 | 2015 |
| plant | Ornithogalum nutans | Drooping Star-of-Bethlehem | 4 | 2014 |
| plant | Ornithogalum pyrenaicum | Bath Asparagus | 840 | 2020 |
| plant | Ornithogalum umbellatum | a Plant | 2 | 2019 |
| plant | Ornithopus perpusillus | Bird's-foot | 41 | 2016 |
| plant | Orobanchaceae | a broomrape or toothwort (unidentified) | 4 | 2017 |
| plant | Orobanchaceae | an Orobanchaceae sp. (unidentified) | 2 | 2002 |
| plant | Orobanche | an Orobanche sp. (unidentified) | 6 | 2009 |
| plant | Orobanche elatior | Knapweed Broomrape | 6 | 2010 |
| plant | Orobanche hederae | Ivy Broomrape | 468 | 2019 |
| plant | Orobanche minor | Common Broomrape | 221 | 2020 |
| plant | Orobanche minor subsp. minor | a common broomrape | 14 | 2008 |
| plant | Orobanche rapum-genistae | Greater Broomrape | 4 | 2008 |
| plant | Osmanthus heterophyllus | an ash | 7 | 2000 |
| plant | Osmunda regalis | Royal Fern | 2 | 2007 |
| plant | Ostrya carpinifolia | Hop Hornbeam | 2 | 2015 |
| plant | Oxalis | a Oxalis sorrel (unidentified) | 30 | 2015 |
| plant | Oxalis acetosella | Wood-sorrel | 519 | 2019 |
| plant | Oxalis articulata | Pink-sorrel | 70 | 2019 |
| plant | Oxalis corniculata | Procumbent Yellow-sorrel | 103 | 2019 |
| plant | Oxalis corniculata var. atropurpurea | a procumbent yellow-sorrel | 2 | 2004 |
| plant | Oxalis debilis | Large-flowered Pink-sorrel | 3 | 2019 |
| plant | Oxalis dillenii | Sussex Yellow-sorrel | 1 | 2016 |
| plant | Oxalis exilis | Least Yellow-sorrel | 4 | 2013 |
| plant | Oxalis incarnata | Pale Pink-sorrel | 4 | 2017 |
| plant | Oxalis purpurea | Purple-flowered Wood-sorrel | 1 | 1985 |
| plant | Oxalis stricta | Upright Yellow-sorrel | 3 | 2019 |
| plant | Oxalis tetraphylla | Four-leaved Pink-sorrel | 1 | 2019 |
| sea-fan, sea anemone or jellyfish | Obelia dichotoma | a Sea-fan/Sea Anemone/Jellyfish | 1 | 1936 |
| sea-fan, sea anemone or jellyfish | Obelia geniculata | a Sea-fan/Sea Anemone/Jellyfish | 1 | 1924 |
| sea-fan, sea anemone or jellyfish | Obelia longissima | a Sea-fan/Sea Anemone/Jellyfish | 1 | 1937 |
| spider | Obscuriphantes obscurus | a money spider | 2 | 1989 |
| spider | Oedothorax agrestis | a money spider | 1 | 1986 |
| spider | Oedothorax apicatus | a money spider | 13 | 2003 |
| spider | Oedothorax fuscus | a money spider | 27 | 2003 |
| spider | Oedothorax retusus | a money spider | 19 | 2009 |
| spider | Oonops domesticus | a six-eyed spider | 4 | 2018 |
| spider | Oonops pulcher | a six-eyed spider | 3 | 1997 |
| spider | Ostearius melanopygius | a money spider | 1 | 1996 |
| spider | Oxyopidae | an Oxyopidae sp. (unidentified) | 2 | 2018 |
| spider | Ozyptila atomaria | a crab spider | 3 | 1989 |
| spider | Ozyptila brevipes | a crab spider | 1 | 1998 |
| spider | Ozyptila praticola | a crab spider | 1 | 2015 |
| spider | Ozyptila simplex | a crab spider | 1 | 2003 |
| spider | Ozyptila trux | a crab spider | 2 | 1985 |
| springtail | Oligaphorura schoetti | a springtail (Collembola) | 1 | 1963 |
| springtail | Orchesella | an Orchesella sp. (unidentified) | 1 | 2015 |
| springtail | Orchesella cincta | a springtail (Collembola) | 21 | 2019 |
| springtail | Orchesella villosa | a springtail (Collembola) | 6 | 2015 |
| tick or mite (acarine) | Oribatida | an oribatid mite (unidentified) | 1 | 2017 |
| true fly - | Ochlerotatus | an Ochlerotatus sp. (unidentified) | 1 | 2011 |
| true fly - | Ochthera | an Ochthera sp. (unidentified) | 1 | 1997 |
| true fly - | Ocytata pallipes | a parasitic fly | 2 | 2001 |
| true fly - | Odinia boletina | a fly | 1 | 2000 |
| true fly - | Odontomyia | an Odontomyia sp. (unidentified) | 1 | 2011 |
| true fly - | Opacifrons coxata | a lesser dung fly | 1 | 1999 |
| true fly - | Opacifrons septentrionalis | a lesser dung fly | 2 | 1996 |
| true fly - | Opetia nigra | a big-footed fly | 3 | 2003 |
| true fly - | Ophiomyia pulicaria | a leaf-mining fly | 4 | 2008 |
| true fly - | Opomyza | an Opomyza sp. (unidentified) | 1 | 2019 |
| true fly - | Opomyza florum | a fly | 11 | 2009 |
| true fly - | Opomyza germinationis | a fly | 52 | 2015 |
| true fly - | Opomyza petrei | a fly | 19 | 2013 |
| true fly - | Ornithomya avicularia | a flat fly | 3 | 2020 |
| true fly - | Ornithomya chloropus | a flat fly | 1 | 1979 |
| true fly - | Ornithomya fringillina | a flat fly | 1 | 1979 |
| true fly - | Orthocladiinae | an Orthocladiinae sp. (unidentified) | 2 | 1991 |
| true fly - | Oscinella frit | a fly | 32 | 2015 |
| true fly - | Oscinella maura | a fly | 2 | 2006 |
| true fly - | Oscinella nigerrima | a fly | 7 | 2008 |
| true fly - | Oscinella nitidissima | a fly | 7 | 2008 |
| true fly - | Oscinella pusilla | a fly | 7 | 2008 |
| true fly - | Oscinella vastator | a fly | 1 | 1983 |
| true fly - | Oscinella vindicata | a fly | 5 | 2006 |
| true fly - | Oscinimorpha minutissima | a fly | 2 | 2008 |
| true fly - | Oswaldia muscaria | a parasitic fly | 1 | 2000 |
| true fly - | Otites guttatus | a fly | 1 | 2000 |
| true fly - cranefly | Ormosia bicornis | a cranefly | 3 | 2001 |
| true fly - cranefly | Ormosia bifurcata | a cranefly | 1 | 2000 |
| true fly - cranefly | Ormosia hederae | a cranefly | 1 | 2000 |
| true fly - cranefly | Ormosia nodulosa | a cranefly | 3 | 2001 |
| true fly - (diptera) | Ophiomyia collini | an insect - true fly (Diptera) | 1 | 2004 |
| true fly - hoverfly | Orthonevra brevicornis | a hoverfly | 3 | 2000 |
| true fly - hoverfly | Orthonevra nobilis | a hoverfly | 7 | 2013 |
| true fly - midge, gnat or mosquito | Oligotrophus betulae | a gall midge | 1 | 1999 |
| true fly - midge, gnat or mosquito | Orfelia fasciata | a fungus gnat | 1 | 2006 |
| true fly - midge, gnat or mosquito | Orfelia nemoralis | a fungus gnat | 5 | 2006 |
| true fly - midge, gnat or mosquito | Orfelia ochracea | a fungus gnat | 2 | 2000 |
| true fly - picture-winged, conopid or stilt-legged fly | Orellia falcata | a gall fly | 2 | 2019 |
| true fly - picture-winged, conopid or stilt-legged fly | Oxyna nebulosa | a gall fly | 4 | 2006 |
| true fly - picture-winged, conopid or stilt-legged fly | Oxyna parietina | a gall fly | 3 | 2013 |
| true fly - robber, empid or dolichopid fly | Ocydromia glabricula | a dance fly | 12 | 2006 |
| true fly - robber, empid or dolichopid fly | Oedalea stigmatella | a dance fly | 3 | 2001 |
| true fly - robber, empid or dolichopid fly | Ogcodes gibbosus | a hump-backed fly | 1 | 2010 |
| true fly - robber, empid or dolichopid fly | Ogcodes pallipes | a hump-backed fly | 1 | 1907 |
| true fly - robber, empid or dolichopid fly | Oropezella sphenoptera | a dance fly | 11 | 2006 |
| true fly - robber, empid or dolichopid fly | Orthoceratium lacustre | a dolichopodid fly | 1 | 1986 |
| true fly - soldier or snipe fly | Odontomyia ornata | Ornate Brigadier | 40 | 2013 |
| true fly - soldier or snipe fly | Odontomyia tigrina | Black Colonel | 35 | 2013 |
| true fly - soldier or snipe fly | Oplodontha viridula | Common Green Colonel | 60 | 2019 |
| true fly - soldier or snipe fly | Oxycera analis | Dark-winged Soldier | 1 | 1948 |
| true fly - soldier or snipe fly | Oxycera morrisii | White-barred Soldier | 4 | 1999 |
| true fly - soldier or snipe fly | Oxycera nigricornis | Delicate Soldier | 12 | 2001 |
| true fly - soldier or snipe fly | Oxycera pardalina | Hill Soldier | 6 | 2001 |
| true fly - soldier or snipe fly | Oxycera pygmaea | Pygmy Soldier | 4 | 2001 |
| true fly - soldier or snipe fly | Oxycera rara | Four-barred Major | 22 | 2015 |
| true fly - soldier or snipe fly | Oxycera terminata | Yellow-tipped Soldier | 1 | 1948 |
| true fly - soldier or snipe fly | Oxycera trilineata | Three-lined Soldier | 8 | 2002 |
| woodlouse | Oniscidae | a Oniscidae woodlouse (unidentified) | 2 | 2018 |
| woodlouse | Oniscus asellus | Common Shiny Woodlouse | 183 | 2019 |
| worm | Octolasion cyaneum | Blue-grey Worm | 22 | 2017 |
| worm | Octolasion lacteum | an annelid | 10 | 2017 |
| worm | Oligochaeta | an oligochaete worm (unidentified) | 135 | 2021 |