SIERA, FIP, xFIP, etc. are pitcher's run prevention stats moon. They isolate pitcher's abilities. ZIPs, Streamer, FGDC and others always protect these estimators year after year, specially FIP.
OPS against is a batted ball stat. It is not a run prevention stat by any means.
If a RP allows three hits in every IP he pitches his OPS against goes up as hell, but he can still hold the inning without a run.
This is why run prevention stats like FIP, xFIP, SIERA are more important than batted ball stats in RPs.