once you refinish you will begin to remove the original stain anyway. This almost has to be done. You can attempt to match the stain you have now with stain charts.
There is a sand sealer that some people use that you apply on the sanded wood before you apply the stain. this sand sealer does allow for a more even and uniform stain. However, I don't know how well it will work on oak or what you have. I find that it works good on pine stain projects.
Anyway, apply this then your stain, then your polyurethane. Consider the pros and cons of water verse oil polyurethane.