- observation post.
- variant of ob- (by assimilation) before p:oppose.
- opera.
- operation.
- opposite.
- opus.
- out of print.
- opus.
- Order of Preachers (Dominican).
- observation post.
- opposite prompt.
- out of print.
- overproof.
- observation post
- Ordo Praedicatorum (the Dominicans)
- organophosphate
- operation
- opus
- operator
- operational
- out of print