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.
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.
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.
There is an unwritten rule that any Original Poster on 4chan is a homosexual. This is a phenomenon that has archaic origins, dating back to the creation of 4chan itself, and possibly back to the IRC-days. This phenomenon can be easily identified, because every other poster is overwhelmingly compelled to point it out.
It is possible that the combination of needing to sustain a single conversational topic, being overwhelmingly charming over a textual medium, finding a suitably enticing and appropriate picture to begin a thread with, and suppressing his instincts to exactly the opposite causes so much stress on the mind that an Original Poster simply becomes homosexual to justify the effort he put in. In being sexually attracted to the necessarily all-male community, the Original Posters brain re-balances their dopamine and serotonin levels naturally, setting right the chemical equilibrium necessary for sanity.
Or they might just be terrible people upon whom are heaped sexual slurs.