I think a 18" O is about as big as u will find a captive O but in the wild i have seen pics that looked like the O could have been 24" but there was no tape measure or anything else for a size comparison.
I think there is A LOT of sensational BULLSH.. out there about oscars. Just two days ago I had this jackass who works at an LFS near me tell me they get FIVE FEET LONG. Anyway - did actually own one that was 15" (of COURSE I don't have pics ). But yeah you hear people saying that there are two foot oscars ALL the time but I've never seen anything even CLOSE to that.
Its no different than those legendary 6 foot RTCs and 3 foot Mbu Puffers that people are always talking about. Folks just WANT TO BELIEVE
If its the one I saw there, then it is in a 75 with a little over an inch of clearance for the bother the head and tail when it turns. Big fish but I think it is 15-16 inches,