Epoxy and Polyurethane compounds are thermoset plastics. Epoxy and Polyurethane substrates bond well to each other as long as the surfaces are clean and slightly abraded.

Can epoxy be used over polyurethane?

It would be better if the epoxy were used on bare wood however it should work over the polyurethane. It would be best to roughen the poly with some 100x sandpaper prior to applying the epoxy. It would make a better bond.

Will epoxy stick to oil-based polyurethane?

You will still need to apply a seal coat of epoxy even though you apply 2-3 coats of oil-based polyurethane. The poly does not seal the wood enough to prevent air bubbles from surfacing into the flood coat.