you should post this on the kitchen's forum, there are many people who have painted existing oak cabinets or gel stained over existing finish and I have seen pics posted there of finished cabinets that were quite beautiful.

We had our oak kitchen cabinets painted a light cream color. We debated about this for four years. My husband was really against painting wood, but finally we just went for it. Now we wish we'd done it sooner. They look amazing. We cannot see the grain underneath. We did have them professionally done. It updated our kitchen...looks great. Go for it, but do it right.

Our oak cabinets were painted over and looked fine. Some grain is visible but it looks perfectly fine IMO. As long as the paint job is well done, I can't understand why people worry about a little grain line. It must be a matter of opinion. I say go for it.