What does OP stand for?
Original Post/Poster.
Seven years here and you didn’t know that?
If you want you can call it “Opening post”. If you want to.
Had a feeling, just wanted to be sure.
I tend to read it as Obsequious Pendant. You’d be surprised how often it fits.
OP = original post, original poster or overpowered, depending on the context
The OP asked what OP stands for in the OP.
[Obsequious Pedant Mode]
I’m pretty sure you meant “Pedant” 
[/Obsequious Pedant Mode]
Going for the low-hanging jokes, I see.
I always use “OP” to mean “Out-Patient”, on the assumption that very few of us are actually, currently, committed.
Optimus Prime
Obvious Pissant
When I was a kid, it meant Ocean Pacific, and we liked it.
I forgot the winky:
The Dominicans: Order of Preachers in English, Ordo Praedicatorum in the original Latin.
Otherwise as noted in Post #2, above.
Opus the Penguin.
A dyslexic’s version of OK.
OP, how can I explain it
I’ll take you frame by frame it
To have y’all jumpin’ shall we singin’ it
O is for Original
The last P…well…that’s not that simple
It’s sorta like another way to call a message or an entry
It’s four little letters that are missin’ here
I’m down with the-O-P…