There is "grain filler" just for that, you can find it at woodworker stores.
But even with that you may still need a good 2K primer to get it completely smooth.
I use "conversion varnish" for wood and it uses a vinyl sealer with the same activator.
It fills pretty good and sands real easy, but for oak you need something more for sure.
Ever wonder how expert wood finishers get the mirror-like finish on fine furniture? It isn't by hastily slapping on a coat or two of standard-issue polyurethane varnish. There are a number of steps involved, beginning with the way the piece is originally constructed.
www.rockler.com