They seen to have a PBAE powertap, because everyone in our group started losing Mana when we fought them with a group, thats why we died. So be sure your Healers are a bit away when you engage them.
Killed him with 2 and half FG's once and had one wipe with just a FG of high RR toons.
Once you have the rest of the room cleared. Have one toon (tank preferrably) run into room and pull him. Once pull has been made have tanks each engage one since all three Oros will come.
Have all the casters and other damage dealers not engaged assist on one of the target, while other two tanks remained engaged to the other two Oros.
I think it is possible to take him out with 1 well balanced FG, just need to make sure you have at least three tanks.(friars and melee tics don't count) We should have had him he was just really uber that time.