1. Lacking seriousness; given to frivolity
   - airheaded, dizzy, empty-headed, giddy, light-headed, lightheaded, silly

See also: frivolous

• Oncorhynchus nerka
• Oncorhynchus tshawytscha
• oncost
• Oncovin
• Ondaatje
• Ondatra
• Ondatra zibethica
• ondes martenot
• one
• one after another
• one after the other
• one and only
• one another
• one at a time
• one by one
• one C
• one dollar bill
• one hundred
• one hundred eighty
• one hundred fifteen
• one hundred fifty
• one hundred fifty-five
• one hundred five
• one hundred forty
• one hundred forty-five