- [x] oak
- [ ] oasis
- [ ] obedience
- [ ] object to
- [x] oblivious
- [ ] obsess
- [ ] obsolete
- [x] obstacle
- [ ] obstruct
- [x] obtain
- [x] occasional
- [x] occupy
- [x] occur
- [ ] oceanographer
- [ ] octopus
- [x] odd
- [ ] odor
- [ ] offensive
- [ ] offspring
- [ ] olfactory
- [ ] on account of
- [x] on behalf of
- [ ] on the basis of
- [ ] on the whole
- [ ] once and for all
- [ ] one after another
- [ ] onslaught
- [ ] ooze
- [ ] opal
- [ ] opaque
- [ ] opponent
- [x] opportunity
- [ ] oppose
- [ ] opposite
- [x] optical
- [ ] optimal
- [ ] optimistic
- [ ] oral
- [x] orbit
- [x] orchestra
- [ ] orchid
- [ ] ore
- [x] organic
- [ ] orientation
- [x] original
- [x] originate
- [ ] ornament
- [ ] other than
- [ ] out of date
- [ ] outcome
- [ ] outfit
- [x] outgas
- [x] outline
- [x] output
- [ ] outrageously
- [ ] outspoken
- [x] overall
- [x] overcome
- [ ] overhaul
- [ ] overlap
- [ ] overlook
- [x] overseas
- [x] overwhelm
- [ ] owing to
- [ ] owl
- [ ] oxide
- [ ] oyster
- [ ] ozone