If you have an old oak floor that was stained very dark, or has darkened with age, you're probably not getting the full visual benefit of the oak grain in the finish. Oak in its natural state is a light-coloured wood, and lighter coloured stains tend to show off the natural wood patterns better than darker ones. The only effective way to lighten a dark oak floor is to sand off the dark stain and refinish it with a lighter one.
Set up your drum sander with 30-grit sandpaper. Run it over the floor from one end of the room to the other in overlapping courses, pushing it forward and back at a moderate pace and going diagonal to the direction of the floorboards. It should take off the top layer of the finish and expose bare wood. (Note: Don't allow the sander to stand in one place while it's running, as this could damage the floor.)
Use a vacuum with floor attachment to take up the dust.
Repeat Step 1, but use 60-grit sandpaper instead of 30-grit, and run the sander in the opposite diagonal direction as before. It should remove the last of the finish and completely expose the bare wood. Vacuum.
Sand the floor a third time, this time with 100-grit sandpaper. Run the sander with the direction of the floorboards for the final sanding. It should get the floor completely smooth. Vacuum thoroughly.
Brush light-coloured wood stain on the floor starting at the corner of the room furthest from the entrance. Brush it heavily over an area of a few square feet, let it sit for one minute, then wipe up the excess stain with a rag. Repeat, working in sections and doing the whole floor. Let it dry overnight.
Brush a coat of polyurethane gloss, starting from the same far corner of the room. Brush it on in a thin, smooth layer, with the direction of the floorboards. Do the whole floor. Let it dry for eight hours.
Dull the surface of the polyurethane with 240-grit sandpaper, sanding it by hand in light strokes with the direction of the floorboards. Vacuum up the dust.
Apply a second layer of polyurethane in the same manner as the first. Let it dry eight hours, sand it by hand, vacuum the debris, and apply a third layer. Let the third layer set for 24 hours before using the floor.
Wear a dust mask when you sand.