Is it true that oil based can go over latex?
You can paint either oil or latex over either kind of primer.
When it comes to painting over paint, then my understanding is that you can paint an oil based paint over a latex paint, but not the other way around unless the oil based paint is a flat paint (in which case it's rough enough for the latex to adhere well).
If you want to paint a latex over an oil, the normal procedure is to apply an alkyd primer first over the oil based paint, and then paint the latex over the primer.
Also, with either kind of paint, you need to rough up the surface of a semi-gloss or gloss paint before painting over it with the same kind of paint. The purpose in doing this is to increase the surface area between the two films to increase the adhesion between them.