I believe that a pick-off is supposed to go down as PK in an actual scorebook, but in virtually all box scores -including those on MLB.com- it is designated as "PO". With the broad title of the quiz and no instruction limiting to scorekeeping/scorebook abbreviations, pick off should be accepted.
Can OBP be modified slightly so that "On base percentage plus slugging percentage" works for OPS? Or "OBP plus slugging" -- I couldn't figure out that I needed to just say "on base plus slugging". Great quiz.