Find me anyone in history where a 91.3 PDO was indicative of an actual sustainable decline? During 5v5 his xGF% is 48.95, 4th among our forwards. And what's this steady decline nonsense. He was near the top in a lot of the Selke metrics last season and finished 4th, and ended the year being arguably our best player in the playoffs. Not sure what your argument would be from his offensive side for a multi season/year decline. Should you give him a massive contract extension? No, just like you shouldn't for any other guy that's about to turn 32, but he hasn't been declining for years.
The big thing with O'Reilly is having the correct linemates. Put a Kyrou or Tarasenko on his line, and it'll be a disaster. Have guys like Perron, Saad, Buchnevich, Schenn, Schwartz, etc., even scrubs like Leivo or Sanford, and he'll excel. He wants guys that play as a group, good on possession/cycle, good in the corners, and someone that is a finisher in that group.