Short for Original Poster. Used on online message boards and forums.
The the message the op posted is idiotic.
http://www.urbandictionary.com/define.php?term=op
Question: What Does 'O.P.' Stand For? What Does It Mean?
At an online discussion forum, you see the phrase "in regards to the OP, I
think that health care is going to be pivotal in this coming election". What
does this OP expression mean?
Answer: 'OP' has two common meanings in the 21st century. It will sometimes
mean 'overpowered' in the world of online gaming. More commonly, OP means
'original poster' in discussion forums. The original poster is the person who
started a discussion thread, and OP is used to refer back to that person as
the responses grow.
http://netforbeginners.about.com/od/internetlanguage/f/What-Is-OP.htm