(informal) Clipping of operation. / (informal) An amateur radio operator. / (Internet) An operator on IRC, who can moderate the chat channel, ban users, etc.

(transitive, Internet) To promote (an IRC user) to an operator.

(Internet slang) Alternative letter-case form of OP (“original post, original poster”).

(slang, among criminal gangs) Alternative form of opp (“opponent; opposition”).

(Australia, informal, transitive) To buy (something) from an op shop. / (Australia, informal, intransitive) To shop at op shops.

(music, abbreviation) Opus. / (abbreviation) Operation. / (abbreviation) Operator. / (abbreviation) Operative. / (abbreviation) A detective. / An operator's license, usually ops

Initialism of Overall Position. / (theater) Initialism of opposite prompt; stage right. / (Internet) Initialism of original poster; the person who started a thread.

(publishing) Abbreviation of out of print. / Abbreviation of overproof. / (video games) Abbreviation of overpowered. / (Internet, music) Initialism of overplayed.

(religious) Initialism of Ordo Praedicatorum (Order of Preachers, the Dominicans, a religious order).

Initialism of opening credits. / Initialism of opening theme.