I searched for many terms like LOL, NSFW and I was able to find out. But the terms like OP or IMO or ATH kind of short forms are seeming very much specific to bitcointalk forum or simply forums.
I guess this topic will help new people if we share abbreviated words with full form.
OP = Original Poster.
IMO is specific for BT? ... which forum you came from?
Honestly, this was one of the first expression I met with as a non-native speaker.
BTW I like WYSIWYG ... It's not a commonly used but it is quite official.
Personally have never used it. I just had to find the meaning when saw it for first time and since then I remember it.