Mana
Issue
Those two words don't belong in the same sentence for the 39 bracket healers.
Disc Priests can get over 4.8k mana easily.
Holy Paladins don't need a lot of mana and the only real way they can OOM is if they spam FoL, but if they are atleast using Judgement when it's not on cooldown then even THAT is hard to do.
Resto Druids will be dead before they oom.
Resto Shamans are similar to resto druids but not as easy to kill as them and their heals are better.
A geared 39 twink healer who's not /facerolling their keyboard can easily keep themselves from OOMing. Out of all my time playing in the 39 bracket I've only oom'd twice on my holy paladin and I've only seen a Disc Priest oom once.