The first and most obvious choice is paint. Using a liquid deglosser or some fine grit sandpaper first, you can increase the quality of adhesion of paint to oak. However, if the cabinets were finished with a catalyzed or even a multi-layer stain and protective coat, getting past the factory or professional finish is easier said than done.
Most of the time you will have to sand the entire outside of the cabinets then stain to your desired color pigment. Followed up with the finish of your choice. Flat, semi gloss, glossy. If your cabinets are venered then you will have to take the venere off and repace with venere of your choice
You can sand to bare wood a re_stain a lighter color or scratch sand and use a liquid sandpaper solution which will dry to the touch ,tac cloth it,then primer and paint with a one coat cabinet paint,many colors to choose. Change your knobs and handles and you got new cabinets for fraction of cost.
You can strip the cabinets and stain them a lighter color, or you can primer and paint them any color you want. This is not a easy project, it requires taking the doors off, sanding them and the cabinets you left on the wall. Not a weekend project. This a two person project.
There are many possibilities. You could paint them with an oil based paint. You could stain them with an even darker colored stain. You could bleach the stain off of the oak and then attempt to restain a lighter color (although this will take much work and probably isn't worth it).