spray poly over oil paint

With Satin paint it is very important to thoroughly mix (several full minutes)  before using and to continually re-mix while applying.  Satin finishes contain a silica flatting agent which must be kept evenly in suspension. 

I do have a question or two. Did you use a primer?  At least two coats of paint?  It's possible that, without primer and/or multiple coats, you are experiencing areas of different porosity in the wood that have let some areas soak in to a greater extent, leaving the surface less shiny.  If this is the problem, it's not entirely certain that the addition of the very thin rattle can finish will give an even sheen. 

Have you considered applying another coat of paint--this time in the semi-gloss sheen.  By the way, if semi-gloss is too shiny, you can split the difference by mixing satin and semi-gloss.  Paint is as tough a finish as the spray clear coat would be, though the aesthetics are different. 

You will want to evenly scuff sand the surface before applying another top coat.  Inspect carefully with lots of light, and use your fingers to feel for defects.  Working toward perfection in the penultimate coat can go a long way to assuring that the final coat gives the desired results.