His run indicator stats were all average. ERA-, FIP-, SIERA, his xFIP was bloated which could be a red flag but that's also a factor of him giving up so many flyballs and xFIP penalizes extreme flyball pitchers. I don't think giving up a bunch of flyballs is inherently bad as long as many of those balls in the air are weakly hit ala Marco Estrada and his pop up rate. Now maybe Oh got really lucky because Busch Stadium is a big park and that likely suppressed his homeruns allowed, but most of his peripherals indicate an average pitcher. He still had a decent K/BB and his SwStr% was also about average for relievers.
It's not like he got f***ing destroyed last season.