Oak is a hardwood with a pronounced open grain. There are two types of stain: semi-transparent (light-bodied) and solid (full-bodied). The semi-transparent stains have less pigment and reveal the grain of the wood better. Solid stains are more durable, hide defects better, and have to be reapplied less frequently. In your case, a semi-transparent oil-based stain would be the best way to fully enjoy the beauty of the oak.

Several coats of varnish should be applied after the stain, making sure to lightly sand between each application. Varnish is oil paint without pigment. Polyurethane is recommended for most uses, because it is clear and dries to a hard surface that will withstand the elements. Additionally, it is available in a satin, gloss, or high-gloss finish. We like Flecto Varathane. If you don't mind a high-gloss look, Spur varnish will outlast them all. By the way... to properly protect an exterior door, it's important to finish all six sides of the door!