What is the best sander to use on oak stair treads?
A random-orbit sander has proven to be one of the most versatile tools I've used, having put hundreds of hours on them for projects of many types. Obviously they have limitations, but with a range of paper grit on hand they're invaluable. Note that this is not the same as a conventional "orbital" sander, which can be a dangerous and damaging instrument in the wrong hands. RO sanders are very forgiving to the novice. 

You may also need a detail sander to get into the corners. These have pointed plates of various shapes that allow very close access, but they're much slower due to their more limited range of motion. 

Finally, your hands. Sometimes the right manufactured tool simply doesn't exist, and a folded up sheet on your highly-evolved digits is the best approach.