If you painted your cement patio with a concrete paint but want to tile over it will the mortar stick to the oil-based paint?

I recommend consulting the flooring manufacturer, however, my experience suggests that the application you describe is an acceptable risk. Adhesion of floor tile to grout is more important than the grout's adhesion to the painted concrete substrate. Uniform stability is very important, and you have that with your painted concrete slab.

When I had my tile done, the company used a primer to rough up the smooth surface.