Golden Mean said:Each creature within the zone doesn’t make d20 rolls to resolve attack rolls, ability checks, skill checks, and saving throws. Instead, these rolls are resolved as if the creature rolled a 10 on the die.
I think it hangs on whether you resolve Oath of Emnity before Golden Mean (in which case you would roll two d20, but instead you pretend you'd rolled two 10s instead) or after (in which case you don't roll at all, so OoE doesn't apply) or rule that they conflict, and Golden Mean wins.
Frankly, I don't think it matters. It's a level 26 power; the PCs are more or less allowed to break the game by that point.