RoundheadRound‧head /ˈraʊndhed/ noun 
someone who supported Parliament against the King in the English Civil War in the 17th century → Cavalier
someone who supported Parliament against the King in the English Civil War in the 17th century → Cavalier- operating theatre
- operation
- operational
- operationally
- operational research
- Operation Desert Storm
- Operation Iraqi Freedom
- Operation Overlord
- operative
- operator
- operetta
- Ophelia
- ophthalmic
- ophthalmologist
- ophthalmology
- opiate
- opine
- opinion
- opinionated
- opinion-makers
- opinion poll
- opium
- opossum
- opp.
- Oppenheimer, J. Robert