Can i paint over oil based polyurethane?

You won't need to sand off all the poly, but do any rough spots and feather them to make the surface smooth. Primer would be ideal, because it will ensure an even coat of color, but it isn't absolutely necessary. If the dinette set is clean and smooth, you can top coat it with a good grade oil base paint, and it will come out looking like a new set. If you use a latex ( water base paint ) it won't adhere as well, and will easily scratch or rub off. Stick with oil base, and you'll be fine.