I'm relatively new to the site and I am sure this is addressed in the readme sections. I've come to realize that lots of people on here use the computer. I use my phone on a web browser. I can't find what OP means. There are many other things that I can't figure out but that one has absorbed me as of late.
"OP" stands for "Original Poster". It's used most often to refer to the asker of a question, as in "We need more clarification from the OP to answer this question".
See also: What does OP mean?