| Orthomegas Audinet-Serville, 1832 |
| Orthomegas cinnamomeus (Linnaeus, 1758) |
| Orthomegas folschveilleri Audureau, 2011 |
| Orthomegas monnei (Bleuzen, 1993) |
| Orwellion Skiles, 1985 |
| Orwellion fasciatum Skiles, 1985 |
| Orwellion gibbulum (Bates, 1880) |
| Orwellion gibbulum subsp. arizonense (Casey, 1891) |
| Orwellion gibbulum subsp. gibbulum (Bates, 1880) |
| Orwellion lineatum Skiles, 1985 |
| Orwellion occidentalis (Giesbert & Hovore, 1976) |
| Osckayia Pérez-Flores & Santos-Silva, 2021 |
| Osckayia oaxacae Pérez-Flores & Santos-Silva, 2021 |
| Osmidus LeConte, 1873 |
| Osmidus guttatus LeConte, 1873 |
| Oxycoleini Martins & Galileo, 2003 |
| Oxycoleus Lacordaire, 1869 |
| Oxycoleus clavipes Lacordaire, 1868 |
| Oxycoleus gratiosus (Bates, 1885) |
| Oxymerus Dupont, 1834 |