and I cannot give a hard and fast answer to that because its a grey area.BUT, seems to me that the staining process is all done and dried and set in. So, that is in concrete if you will. Then the oil poly is on top of a dried and set stain. So, my opinion, is that I see no problem.
My basis for my conclusion is that its two steps, the first step is applied and dried out and part of the wood. The second is on top of that.