I'm pretty sure you have to sand it before you do anything else (like paint or oil it) so maybe try that?

If you havent already done it yet, the best way is too try find a Cabinet maker or someone around that has a Drum Sander, they roughly cost about $30-50 to put you wood through the level out all the bumps and rough edges, if you can't find one the the next easiest quickest way is to use a Belt Sander. Once it is flat then use and Orbital Sander and sand up to 240 Grit also dont forget to sand the Live Edge sectiom. You can get away with then Varnishing and oiling it after that, but i normally tend to go 600+ even have sand paper at 2000 grit. If i varnish it instead of oil i then use steel wool to smooth it out a bit. I have attached pictures of some live edge Bar tops and normal edge mantel we have done in the past

Bar Tops · More Info

Raintree Mantel for Outdoor Living Area · More Info