1. Having a determined will
   - strong-willed 
2. Marked by vigorous independence of thought and judgment

See also: independent, obstinate, stubborn, unregenerate

• oophorosalpingectomy
• oops
• oops-a-daisy
• Oort
• Oort cloud
• oosphere
• oospore
• ootid
• ooze
• ooze leather
• ooze out
• ooze through
• oozing
• oozy
• op
• op art
• op shop
• op-ed
• op. cit.
• opacification
• opacify
• opacity
• opah
• opal
• opal glass