I thought "OP" stood either for
"Out Patient" (as in some posters here need to get treated)
or, alternatively:
"Old Poster" (as in over the hill and due for a cull!)
(Is this a dig? Well sort of - I have noticed that "R***" often these days has a sideways swipe at "S******", so I thought I'd even up the odds a bit!)
Come on folks, the motto of these boards should be: "help with humour & no malice"