Yep... killing oakenmir gets you lots of XP. My assassin who did it raised some thirty levels as well -- it was downright annoying as I levelled up.
Sadly, the game does not bring the impact of your oath into play, or there would be some consequence for this. We are pretty sure that the developers wanted there to be an impact, but the game was rushed out of development for budgetary reasons (like there wasn't much budgetary left
).