Can I Use Oil Paint Over Water Paint?

Only if the water based paint is flat paint. If the water based paint is semi-gloss, gloss, satin latex, then the oil paint will not stick.
You will need to prime the area to be painted first with the appropriate primer.
Once you use an oil based paint, if you ever want to go back to a water based paint, you will have to prime again.
  If you are wanting a slick finish, (which is what you will get with an oil based paint), you could use a latex gloss or semi-gloss instead of an oil based paint.
It's also a much easier clean up.