People all over the Internet have asked the same thing—which, on the face of it, suggests that different forum posters (and perhaps, in some cases, the same people at different times) have used it to mean both things.
Does OP mean Original Post or Original Poster or both? (AnandTech, posted January 7, 2005)
Does OP mean the original post or poster? (Physics Forums, posted January 3, 2008)
Does OP stand for "Opening Post," or "Opening Poster?" (Kongregate, posted May 7, 2011)
What does OP mean? (Meta Stack Exchange, posted September 10, 2012)
Noob Question: What does OP mean? (World of Warcraft Forums, posted September 15, 2012)
what does op mean? (League of Legends Forum, posted January 3, 2013)
OP -- Original Post or Original Poster? (The Straight Dope, posted December 12, 2016)
If you examine these various discussions, you'll see that, typically, someone will say that OP stands for original poster, and then someone else will say that it stands for original post, and then someone else will say that it can be used to mean either thing, and then someone else will suggest that it stands for something off the wall like Otto Preminger or Opossum Party.
At English Language & Usage, I think, most longtime participants use OP as an abbreviation of original poster, but not everyone follows this convention all the time.
Perhaps the most neutral, descriptivist handling of the abbreviation that I've come across is the one in Daniel Chandler & Rod Munday, A Dictionary of Social Media (Oxford, 2016):
op 1. (OP) In online forums, short for original poster (who started a thread) or original post (the message that started it).