No, the points you earn in Onslaught do not count on your online stats.
The problem is that if they did you would just get people padding away on easy difficulty - not trying to capture objectives just getting easy kills and umpteen bazillion easy points. Thereby making the online stats pointless.
I think it is better this way.
its fun realy.. its not for the xp its just for fun.. thats kinda why they made it.. to get together as a team and kick some AI butt. and i love it ^_^ its just a break from online realy, you dont have to have xp for eveything single little thing you play on that game you know...