Sweeney, latex over oil is no problem. Oil over latex can be. If the oil has been up for a long time--several years or more--usually you will not have a problem but I would lightly sand to give a little tooth for the new paint to stick to.
More problems come from poor prep work than the paint. What I do is wash with Tri Sodium Phosphate and rinse, sand if oil paint and do any spackling, wash again with TSP, prime, paint. Good prep work will take a lot longer than just rolling on a coat or 2 of paint but will give you a better job now and save you a lot of problems in the furure. Henry in MI