OP

An operation.

Example: "My mother's going in for her knee op today."

An amateur radio operator.

An operator on irc, who can moderate the chat channel, ban users, etc.

Op art; a style of abstract art.

Example: "An op painting; op artworks"

Initialism of overall position.

Initialism of online pharmacy.

Initialism of opposite prompt; stage right,

Initialism of original poster; the person who started a thread.

Example: "I don't think your response really answers the op's question."

Initialism of original post.

Initialism of opening theme.

A lookout or vantage point

The credits shown at the beginning of a film, tv program, etc.

Any ester of phosphoric acid or its derivatives, especially one used as an insecticide or herbicide.