What is the best way to paint over oil based enamel interior paint.?

Always use a primer when covering enamel paint with latex. I use Bullseye 1-2-3 and always had good results. Do not use the primer and the latex mix.

Do the primer first and then a good quality latex. It is more work but gives you far better results and if you do not want to do this over again in a short period of time then the two step is the way to go. .

Stop the oil based paint down first. Painting over can be more of a hassle and come out uneven or fish eyed in spots.

Paint over it with white flat paint

Primer with a light color coat, then paint, the oil base will adhere better.

Clean the item using TSP or soap and water. Wipe dry. Light sand the surface using 220 or 320 grit sandpaper. Vacuum off or wipe down the surface with a clean, dry cotton towel or rag. The aim is to get rid of as much as possible. Next use a tack cloth to finally prepare the surface, but without further embedding the dirt into the surface. Apply a coat of quality primer (tinted the same as your latex paint). Apply one to two coats of the latex paint.