Epoxy will adhere to just about anything but we do not recommend using epoxy over oils or oil stained finishes. Oil finishes tend to create a barrier between the wood and the epoxy which doesn't allow the epoxy to adhere to the surface. If an oil-based stain has been used, you must ensure the stain is completely dry and a 2 part/water-based polyurethane is applied before proceeding with the flood coat step. This will create a barrier between the oil stain and the epoxy.
We recommend lightly sanding the surface with #220 - 320 grit sandpaper to allow the epoxy to adhere properly. This also removes any foreign elements that could create problems when applying the epoxy later.
Using a clean rag and denatured alcohol, wipe down the surface to remove any loose residue from the finish. Allow the surface to dry and repeat this step until residue doesn't appear on the rag.
If any residue is left on the surface, this can create a contamination with the epoxy and prevent the epoxy from adhering to the surface properly.
If you are ever unsure about your application, we recommend doing a small scale test to ensure compatibility before final application.