OP

OP is an acronym that is seen frequently on the internet wherever games are being discussed and also with regularity on anonymous imageboards like 4chan. This is because OP can potentially mean one of two things: either overpowered or original poster. Both meanings can found used on /tg/, and only painstaking forensic examination of the context stands a chance of clueing you in as to which meaning was intended. Don't be upset if you get things wrong the first few times; this is a difficult concept, and even many veteran posters struggle with it from time to time.

Overpowered

Sometimes, game designers get things wrong (or at least as far as you're concerned they did it wrong and you clearly know better) and they accidentally/deliberately make an unbalanced game such that some options, units, or characters are significantly more powerful than their counterparts, usually without a fair balancing factor in terms of cost. A prime example would be Clerics in 3rd edition D&D, who are significantly more combat-capable than most other classes (even the classes dedicated to combat) by virtue of their spells and other abilities. Thus, a poster may complain that "Clerics are OP".

A good check for when this meaning is intended is to analyse the post and determine if "OP" is being used as an adjective. If it is also used with a preceding qualifier ("ridiculously" is a common one) you can be almost certain the poster meant "overpowered", and can berate them for their lack of mechanical understanding safely.

Not to be confused with overpowdered, which is what Rouges are.

Original Poster

The Original Poster of a thread is the chap that posted the very first comment and started the thread in the first place. Though this expression is meaningful on any internet forum, it is mainly used on 4chan and similar anonymous imageboards. Use of the term arose because, on a mainly anonymous board such as 4chan, the original poster will usually have no associated name by which to refer to them, remaining anonymous; hence if a later poster has a need to refer to the person that started the thread, they will do so by calling them the OP.

If you find OP being used as a noun, it is a good bet that this meaning was intended. Use of the definitive article ("the OP") is an almost certain giveaway.

Some forums use the acronym TC (topic creator), which has the same meaning.