Ive used oats + whey protien, as a meal replacement shake.
As for PWO, can't say I have, but thats not to say its not a bad idea. I think it depends on goals/persons metabolism, etc.
If your cutting, maintaining, or going for lean mass only, I think its a pretty good idea.
For the traditional bulk, I think simple carbs is a better idea.
Only thing is I believe the whole idea of simple carbs PWO is because insulin reduces cortisol levels (which as im sure you know skyrocket after exercise, and cortisol leads to muscle loss). My only concern would be that the oats/whey mix doesn't elevate insulin enough to lower cortisol levels.
I dunno if that made sense, it was just a thought. Someone correct me if that statement isn't right.