| Ochrophyta | see Heterokontae |
| Onychophora | EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>PROTOSTOMATA>ECDYSOZOA>PANARTHROPODA>ONYCHOPHORA |
| Oomycota | EUKARYA>CHROMALVEOLATA>HETEROKONTAE>OOMYCOTA |
| Opalinata | EUKARYA>CHROMALVEOLATA>HETEROKONTAE>OPALINIDA |
| Opisthokonta | see Unikonta |
| Orthonectida | EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>METAZOA>BILATERIA>PROTOSTOMATA>LOPHOTROCHOZOA>EUTROCHOZOA>ORTHONECTOZOA |
| Osteichthyes | EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>VERTEBRATA>GNATHOSTOMATA>OSTEICHTHYES |
| Osteostracomorphi | EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>VERTEBRATA>OSTEOSTRACOMORPHI |
| Oxyphotobacteria | EUBACTERIA>OXYPHOTOBACTERIA |