1. The ordinal number of forty-three in counting order
   - forty-third

See also: ordinal

• 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
• family Oxyuridae
• family Paeoniaceae
• family Paguridae
• family Palaemonidae
• family Palinuridae
• family Palmaceae
• family Palmae
• family Pandanaceae
• family Pandionidae
• family Panorpidae