The letters "OP" can have a lot of different meanings.

As an initialism, OP can mean "Original Post" or "Original Poster", referring to the start of a conversation, or the person who started that conversation. For example, deep into a long exchange, someone might write "the info is in the OP", suggesting you go back to the message that started it all, or "turns out OP was wrong", implying that the person who started the conversation has since been proven wrong.

As a shorthand, OP can mean "Overpowered", referring to something which is far too powerful. It can be a video game item which makes gameplay too easy, a movie character which is too strong to have any believable threats, or anything else perceived as badly designed due to its extreme power level.

Although the two meanings above are the most common ones, you might also encounter OP as a compliment meaning "On Point", as a shorthand for an "Opening theme", and as many other less common different things.