Both white oak and red oak are blonde woods with delicate wood grain patterns, and oak's soft, smooth texture is best protected by a natural-looking finish. Traditionally, artisans favored either clear or amber shellac as a finish for interior doors because it offers protection but won't obscure the wood's natural texture and color, unlike some modern petrochemical or plastic-based finishes. After a little surface prep, you can apply a multicoat shellac finish in a single day without the hassle of removing the door from its frame.
Remove the doorknob using a screwdriver; set the knob aside in a safe place.
Sand away any roughness or raised grain on the door using 80-grit sandpaper. Be sure to rub in the direction of the wood grain, starting from the top and working down. Wipe off any sanding residue using a clean shop towel or lint-free cotton rag.
Polish the door to a higher level of smoothness using a 180 grit -- or finer -- grade of sandpaper. As before, rub in the direction of the wood grain. Remove any residual sawdust using a tack cloth.
Fold a clean, lint-free cloth into a roughly 4-by-4-inch pad. You can use an old, but freshly washed, cotton T-shirt or a new, unused shop cloth.
Slide a plastic or canvas drop cloth under the door to absorb any drips.
Slip on a pair of rubber or vinyl gloves. Soak the pad with shellac, and then begin wiping it into the wood. Start from the top and gradually move downward, rubbing in the direction of the grain. You can stop rubbing when you can see that the wood is soaked with the shellac. Wipe off any excess material so only a thin coating of shellac remains. After you've finished one side of the door, apply the shellac to the other side, and then move on to the door's edges. Allow the finish to dry for one hour.
Rub in a second coat of shellac on all sides of the door using a fresh wiping cloth. Allow the finish to dry for one hour, and then apply a third coat of shellac.
Re-attach the doorknob after the third coat has dried completely, which should not take longer than two hours in normal indoor conditions.