is short for original poster
.com and elsewhere online.
On Reddit the OP can be referring to different posters (Reddit users) but most often refers to the person who posted the actual Reddit thread that the person saying OP is posting in. It often refers to the first poster within a nested set of comments as well. Often it causes confusion as to which OP the person is referring to, when sometimes the poster directly above a given post is referred to as the OP. It's usually pretty easy to tell who the OP is that is being referenced based on context. Did that confuse you, sorry, the over use OP confuses everyone.
There is also the term "OP delivered" - wich refers to the OP (orignal poster) doing something usually interesting or suprising, or what they said they'd do.
An example would be the poster who took a video of himself 'eating a dick' after saying he would do so if another poster received 400 gildings of Reddit Gold - that OP delivered.
Currently the top upvoted OP on Reddit is user Lumpawarroo, with 66,394 for the post:
Theory Jar Jar Binks was a trained Force user, knowing Sith collaborator, and will play a central role in The Force Awakens
in the subreddit /r/starwars
(the 2nd most upvoted has 56,000 upvotes but that's because the post was testing a new feature on Reddit, the most legit 3rd most upvoted post of all time has 38,500 upvotes so you can see how much higher #1 is)