OP is a phrase that gets tossed around quite a bit, usually against well-liked characters who absolutely wreck house in multiplayer modes within video games. When used in reference to a game’s character, bosses, weapons, or mechanics, it means overpowered. How do fans determine if something is overpowered? If it has much fewer weakeness than strengths, of course! An OP character usually requires a very specific scenario to be defeated as it runs amok, beating up everyone else with ease. Usually, the decision that something in a game is considered OP is followed by the call for nerfing and re-balancing of all of the characters.
Recent examples of OP elements in games are Torbjorn from Overwatch, the Gjallarhorn or Vex Mythoclast from Destiny, and the Vector from The Division. They put players in the position to control a game simply because nobody else was using the character/weapon, as well.
There is one other way you’ll hear the term OP uttered, and it applies to video game forums. In that case, the two letters stand for Original Post or Original Poster (the person who wrote the post). It will usually come up when a conversation is becoming derailed and someone wants to get back on topic.