I swore to not leave this boss alive before Legion.

Honestly easier than I thought. Gear does 50% of the kill (I had 23% Leech). Surviving P1 is obviously the hardest part. Here's how I did it :

At pull, use all cooldowns. I play with Prismatic Crystal for maximum DPS.
Do not use anything for the first Acid Torrent.
Start kiting, then use Ice Block on the second Acid Torrent (you cannot survive even with only 1 stack)
NEVER KICK BLACKROCK BARRAGE. It makes Oregorger's energy drop much faster (-5 every cast).
Use Alter Time on the next Torrent, then Cold Snap + Ice Block for the next.
Eat the last one, and P2 should start on the next Blackrock Barrage.

P2 is easier if you can survive the 100 stacks of Rolling Fury. Managing cooldowns is essential : always use ring and Combustion on Darkmoon proc.
Prismatic Crystal use in P2 is a bit tricky, as Oregorger moves too fast to use it normally. I just save a Hot Streak and 2 Blast Wave, then place PC where Oregorger is about to land a throw everything I can before he moves.

Alter Time is a critical tool in P2 : only use it when the boss is about to get out of reach. Wait for Ice Block cooldown, and do not use Cold Snap before IB is back off cooldown, or you might die.

All in all, a pretty fun boss, the most painful part of the run is the double vanish corpse run :D

Music :

PLAMA - Leaving
SleeplezZ - Euphoria

Patch 6.2.4, 738 ilvl, 23.37% Leech.

Enjoy :)