Oh why ever would you want to paint over oak Pejay747? Couldn't you just get rid of the gloss varnish & matt varnish & wax it, making it look more natural & richer? That's only my opinion of course!
Yes you can if you so wish - rub the surface down with an abrasive and ensure that all the dust is removed then paint away. Ensure that you use a suitable paint and check the back of the tin for further instructions.