Yes, you can use latex paint over oil-based paint. It helps to scuff down the surface of the oil paint layer for improved stickability. I think that any kind of paint-over-paint project is a cause for concern. Even using using similar types of paint, you obviously won't get as good adhesion as you would by laying down paint upon primed wood. But that's reality. Untold layers of dust and dirt, too, will compromise the stickability. Be sure to wipe down with TSP (the painter's best friend) to clean the surface after you do the sanding.
Finally, we do have it on the authority of Rohm and Haas, a Dow Chemical Company, which says:
"The rule of thumb is that, given proper surface preparation, for exterior use you can apply quality latex paints over oil-based, but not the reverse. However, if you have many layers of oil based paint, stick to using oil on oil. For interior use, generally you can use one over the other. Some manufacturers of latex products will recommend a primer when going over oil-based paint."