A: Generally speaking, you shouldn't try, but depending on the scenario here are some tips.
If you are going to paint previously stained woodwork you should really consider hiring a professional. There are a lot of steps to take to ensure the primer and top coat adhere. It is time consuming and smells bad. I've written previously about it but generally speaking, it should be done by someone who has done it many times before. If done well, you can make stained oak look great. If rushed, you will regret it.
With that said if you have previously painted trim work and just want to brighten up, you can use a product by the name of "Advanced" by Ben Moore. I like it because the luster of the finish is like an oil and it lays down much nicer than a lot of the latex enamels.
Prior to the top coat however, you should take these minimum steps.
1. Scuff the surface. Use a medium purple sanding sponge. It should chalk the surface.
2. Vacuum the surface. Use a brush on the end of the hose.