Definition from Wiktionary, the free dictionary
- (Internet) original poster
- I don't think your response really answers the OP's question.
- (Internet) original post
- (military) observation post
- (publishing) out of print
- (religious) Order of Preachers, the Dominicans, a religious order
- Overall Position
- (video games) overpowered (adj.)
- online pharmacy
- (organic chemistry) organophosphate
See also 
- (surgery) Abbreviation of Operation.
- (surgery) Abbreviation of Operationssaal.
- (electronics) Abbreviation of Operationsverstärker.
- (slang, television, film) opening credits and title roll sequence