OP, O.P., or Op may refer to any of the following:
- Observation post, a position from which soldiers can watch enemy movements
- Ocean Pacific, a retail company based in California, United States
- Orange pekoe, a grade of black teas
- Ordo Praedicatorum, a Catholic religious order
- Operating profit, a measure of business profitability
- Out of print, a status of a book title at a publishing house
- Op, or grow-op, a marijuana growing operation
- Open pollination
- Mathematics
- Operator (mathematics), a mapping from one vector space or module to another
- Op (statistics), a symbol used in probability mathematics
- Octagonal prism, a prism formed by square sides and two regular octagon caps
- Culture
- Op art, a style of visual art that makes use of optical illusions
- Op-ed, a newspaper opinion piece of a writer unaffiliated with the editorial board
- Opposite Prompt, stage right on a theatre stage
- Op., an abbreviation for Opus number, used to denote one work of musical composition from many
- Optimus Prime (Transformers), a character from the Transformers franchise
- One Piece, a Japanese manga and anime
- A shortening for opening credits, especially in anime fandom
- Internet and gaming
- Original post or original poster, the first post or person to post on a topic in an online forum
- Overpowered, in video games, noting that a particular aspect or character is unbalanced and thereby too strong
- An Internet Relay Chat operator
- Education
- Oratory Preparatory School, a Catholic preparatory day school for boys in New Jersey, United States
- Overall Position, the grade appointed to Queensland high-school graduates
- Old Persean, an ex-pupil of The Perse School, Cambridge, England
- Old Pauline, an ex-pupil of St Paul's School, London
- Old Pangbournian, an ex pupil of Pangbourne College
- Geography
- Oak Park, Illinois
- Oakland Park, Florida
- Orchard Park, New York
- Overland Park, Kansas
- Ohio Penitentiary, a now-demolished prison
- Science and medicine
- The oropharyngeal airway, leading from the mouth to the pharynx
- Organophosphate, the general name for esters of phosphoric acid
- Osteitis pubis, a medical condition which is prevalent among players of Australian rules football
- Osteoporosis, a degenerative disease of the bones
- A surgical operation