I actually agree with psymed, oak is not dead until he is dead dead, so you should not get xp for killing him until he's actually gone... Last Stand seems like its designed to allow oak to get a kill on you before he dies... if you let him do this, then I don't think you should be rewarded for this
Okay. Last stand. Oak runs really fast, and hits really hard. Tends to kill a lot of people. When he falls regardless because he's already dead, the player gets credit for a kill, but no XP. Thats an imbalance issue, and should be addressed.