I want to paint my oak cabinets a dark color, but I am not sure how this will look.
A dark color could be a good choice if this appeals to your senses and goes with your bathrooms decor. A good way to check out a color before application is to choose a color and paint a sizable piece of cardboard. The cardboard could be in front of the cabinet for a visual inspection.
Paint is just one option. Your cabinet could be restained a darker color with a gel stain. This would still maintain most of the wood grain but push the color to a darker or more striking color.
Always test a color or idea before going through the work of application. This way you won't be disappointed.