An operation.
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.
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.
I don't think your response really answers the OP's question.
Initialism of original post.
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.