1. Perennial of the eastern United States having early solitary yellow flowers followed by late petalless flowers; so-called because ice crystals form on it during first frosts
   - frostweed, frost-weed, frostwort, Helianthemum canadense

Type of: helianthemum, sun rose, sunrose

• genus Orontium
• genus Ortalis
• genus Orthilia
• genus Orthopristis
• genus Orthotomus
• genus Orycteropus
• genus Oryctolagus
• genus Oryx
• genus Oryza
• genus Oryzomys
• genus Oryzopsis
• genus Osmanthus
• genus Osmerus
• genus Osmunda
• genus Ostrea
• genus Ostrya
• genus Ostryopsis
• genus Otaria
• genus Othonna
• genus Otis
• genus Otus
• genus Ouranopithecus
• genus Ovalipes
• genus Ovibos
• genus Ovis