1. By legislation
   "legislatively determined"

See also: legislative

• family Oleandraceae
• family Onagraceae
• family Oniscidae
• family Ophidiidae
• family Ophiodontidae
• family Ophioglossaceae
• family Opisthocomidae
• family Opisthognathidae
• family Orchestiidae
• family Orchidaceae
• family Orectolobidae
• family Oriolidae
• family Ornithorhynchidae
• family Orobanchaceae
• family Orycteropodidae
• family Oscillatoriaceae
• family Osmeridae
• family Osmundaceae
• family Osteoglossidae
• family Ostraciidae
• family Ostraciontidae
• family Ostreidae
• family Otariidae
• family Otididae
• family Oxalidaceae