(computing) An instruction or operation that has no effect; a null operation.

(LGBT, of a transgender or transsexual person) Having chosen not to undergo sex reassignment surgery.

(LGBT) A transgender or transsexual person who has chosen not to undergo sex reassignment surgery.

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

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

Initialism of opening credits. / Initialism of opening theme.

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).

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