If you use oak, I would suggest filling the grain first with oil based grain filler. Oak has a very open pore structure (you can blow smoke through an oak board) that will not take the finish you apply and leave it open to the elements. Oak unfortunately has a tendency to turn black when in contact with water, even if the contact is brief. You will also be very surprised at how much better the finish looks.
I'm a professional cabinet maker, and we always fill oak, ash, walnut and mahogany when we make furniture with it. As far as a finish is concerned make sure you use polyurethane with a UV inhibitor, commonly sold for boat finishing. If you have access to spray gun set up, the finish will be much better. Be sure to sand with 220 grit between coats.
