1. A branch of the Tai languages
   - Thai, Siamese

Type of: Tai

• genus Omphalotus
• genus Oncidium
• genus Oncorhynchus
• genus Ondatra
• genus Oniscus
• genus Onobrychis
• genus Onoclea
• genus Ononis
• genus Onopordon
• genus Onopordum
• genus Onosmodium
• genus Onychium
• genus Onychogalea
• genus Onychomys
• genus Opheodrys
• genus Ophiodon
• genus Ophioglossum
• genus Ophiophagus
• genus Ophisaurus
• genus Ophrys
• genus Opisthocomus
• genus Opuntia
• genus Orbignya
• genus Orchestia
• genus Orchis