Oberyn as a monk? Mountain as a barbarian? Really? Did you watch the series? Or read the books?
Yes. Doesn't mean we agree with you on things.
Both are fighters, period. Maybe Oberyn has some levels in rogue or alchemist for his background in poisons and at The Citadel, but nothing else.
Oberyn does nothing Fighter-y at any point. Swashbuckler all the way, man. Plus some Ninja for skills and poison use, anyway.
And the Mountain clearly has some Fighter levels, but given his psychotic rages and general toughness, some Barbarian also seems appropriate.
I'm not discussing from a metagaming/mechanics point of view, but from a background point of view.
Ok, I give it to you that Oberyn is a rogue, he was always roguish in his past endeavours. But ninja?
And The Mountain is a noble from Casterly Rock, he is a knight, has a castle, lands and men. How could he be a barbarian? Also, his rage problably comes from a injury in his head, which gives him some real bad headaches and makes him drink too much fleur-de-lys.
But, if RPGs are just plain miniature/board games to you, his toughness could be explained with the feat of the same name, or other mechanics that don't involve him being a barbarian.