OP - Original Poster
OP: Original Poster (like you explained) or Original Post.
OP can be used like two words: Topic and Thread. Many posters don't know difference between Topic and Thread as well as they can differentiate what OP implies for either Original Poster or Original Post. It must be taken in to a specific context.
OP, what is it?Terminology'OP'
Original Post / Original Poster. This could refer to either the user that started the thread or the first post in the thread. If you are in the hardware section then it could also mean 'Over Power'.
Add it to what you people have said, in the forum context and also etymological content of the meaning OP are those two meaning of tranthidung but we can also take to the field of linguistics which can mean Operator/Operation or Overall Position. And also OP can be used differently in different field of usage. But as for the forum in different boards with different uses. Like has tranthidung said.