• Species and lower taxa
• Higher taxa
• Common names

Obelia sp.

Oceania armata

Octobulbacea montehermosensis

Octocanna haeckeli

Octodendron cubocentron

Octophialucium bigelowi

Octopoteuthis sp.

Octopus burryi

Octopus defilippi

Octopus macropus

Octopus vulgaris

Octopyle stenozona

Oculosetella gracilis

Ocypode quadrata

Ocyropsis crystallina

Ocyropsis maculata

Ocythoe tuberculata

Oikopleura albicans

Oikopleura cophocerca

Oikopleura cornutogastra

Oikopleura dioica

Oikopleura drygalskii

Oikopleura fusiformis

Oikopleura gaussica

Oikopleura gracilis

Oikopleura intermedia

Oikopleura longicauda

Oikopleura parva

Oikopleura rufescens

Oikopleura valdiviae

Oikopleura weddelli

Oithona atlantica

Oithona attenuata

Oithona bjornbergae

Oithona brevicornis

Oithona fallax

Oithona frigida

Oithona hamata

Oithona hebes

Oithona minuta

Oithona nana

Oithona oculata

Oithona oswaldocruzi

Oithona plumifera

Oithona pseudofrigida

Oithona rigida

Oithona robusta

Oithona setigera

Oithona similis

Oithona simplex

Oithona tenuis

Oithona vivida

Olindias phosphorica

Olindias sambaquiensis

Ommastrephes bartramii

Ommastrephes pteropus – synonym of Sthenoteuthis pteropus

Ommatartus tetrathalamus – synonym of Didymocyrtis tetrathalamus

Oncaea antarctica

Oncaea atlantica

Oncaea clevei

Oncaea curta

Oncaea curvata

Oncaea englishi

Oncaea lacinia

Oncaea latimana

Oncaea media

Oncaea mediterranea

Oncaea parila

Oncaea prolata

Oncaea scottodicarloi

Oncaea venusta

Oncaea waldemari

Onchocalanus magnus

Onchocalanus subcristatus

Onchocalanus trigoniceps

Onchocalanus wolfendeni

Onychoteuthis banksii

Onykia carriboea

Opecarcinus hypostegus

Orbulina universa

Orcadia riedeli

Orchistoma collapsa

Orchistoma pileus

Orchomenella abyssorum

Orchomenella distinctus

Orchomenella gerulicorbis

Ormosella acantharus

Ormosella cornucopia

Ormosella haeckeli

Ornithoteuthis antillarum

Osachila antillensis

Osachila tuberosa

Otosphaera polymorpha – synonym of Solenosphaera chierchiae

Otosphaera polymorpha – synonym of Solenosphaera zanguebarica

Ovalipes catharus

Ovalipes punctatus

Ovalipes trimaculatus

Oxycephalus clausi

Oxycephalus latirostris

Oxycephalus longipes

Oxycephalus piscator

Oxygyrus keraudreni

Oxytricha saltans – synonym of Actinotricha saltans