Original Poster. Common Internet forum usage, not a uniquely military term.what does the green OP mean or when referring to someone? I do not see an acronym nor have I managed to work that one out...
If it’s green, they are logged in at that moment.
If you do @username in the text of your post, you ensure username is notified about it - doesn’t matter if they are OP, a poster in the thread of interest, or not. It flags it for their attention.