Ochthochloa compressa (Forssk.) Hilu – synonym of Chloris flagellifera (Nees) Peterson
Oplismenus crus-pavonis Kunth – basionym of Echinochloa crus-pavonis (Kunth) Schult.
Oropetium aristatum (Stapf) Pilg.
Oropetium minimum (Hochst.) Pilg.
Oropetium thomaeum (Linn. f.) Trin.
Oryza angustifolia C.E. Hubb. – synonym of Leersia nematostachya Launert
Oryza brachyantha A. Chev. & Roehr.
Oryza glaberrima longiglumis A. Chev. – synonym of Oryza glaberrima Steud.
Oryza longistaminata A.Chev. & Roer.
Oryza tisserantii A. Chev. – basionym of Leersia tisserantii (A. Chev.) Launert
Oryzopsis holciformis (M. Bieb.) Hack. – synonym of Piptatherum holciforme (M. Bieb.) Roem. & Schult.
Ottochloa arnottiana (Nees) Dandy – synonym of Ottochloa nodosa (Kunth) Dandy
Ottochloa letouzeyi Renvoize – synonym of Ottochloa nodosa (Kunth) Dandy
Ottochloa nodosa (Kunth) Dandy
Oxyrhachys gracillima (Baker) C.E. Hubb.
Oxytenanthera abyssinica (A. Richard) Munro
Oxytenanthera albociliata – synonym of Pseudoxytenanthera albociliata