| Orobus polymorphus (Nutt.) Alef., 1861 |
| Orobus sericeus Sessé & Moc., 1890 |
| Orobus trifoliatus Sessé & Moc., 1894 |
| Otholobium C.H. Stirt., 1981 |
| Ottleya Sokoloff, 1999 |
| Ottleya greenei (Wooten & Standl.) D.D. Sokoloff, 1999 |
| Ottleya mollis (A. Heller) D.D. Sokoloff & Gandhi, 2003 |
| Ottleya nivea (S. Watson) D.D. Sokoloff, 1999 |
| Ottleya oroboides (Kunth) D.D. Sokoloff, 1999 |
| Ottleya plebeia (Brandegee) D.D. Sokoloff & Gandhi, 2003 |
| Ottleya rigida (Benth.) D.D. Sokoloff, 1999 |
| Ottleya strigosa (Nutt.) D.D. Sokoloff, 1999 |
| Ottleya utahensis (Ottley) Sokoloff, 1999 |
| Oxyrhynchus Brandegee, 1912 |
| Oxyrhynchus alienus Piper, 1924 |
| Oxyrhynchus populneus (Piper) Norvell ex A. Delgado & E. Estrada, 2010 |
| Oxyrhynchus trinervius (Donn. Sm.) Rudd, 1967 |
| Oxyrhynchus volubilis Brandegee, 1912 |
| Oxytropis DC., 1802 |
| Oxytropis acutirostris (S. Watson) M.E. Jones, 1895 |