I think you might be right that there is some nonzero difference in the pronunciation of words containing (o)う vs (o)お sometimes, but it's not pronounced as a p

I think you might be right that there is some nonzero difference in the pronunciation of words containing (o)う vs (o)お sometimes, but it's not pronounced as a proper "ou" most of the time; if you need to pick one, you'd go with straight "oo". I just went through a bunch of examples with a Japanese friend and we can't really find a consistent difference between おう and おお, but it seems we agree that trying to pronounce either too far into "ou" land sounds wrong... at least some of the time... It's kind of tricky :-)