Tung oil has been used as a wood preservative in China for centuries, it is also the main ingredient of many oil based varnishes.
Tung oil will enrich the grain and form a hard, water repelling layer approximately 5mm deep (deeper if you let it down with a bit of camellia oil). Be sure to oil both sides of the board so that you don't introduce cupping.
Pure natural tung oil does take time to dry (no chemical accelerants), but has the advantage of being non toxic and food safe. We recommend giving it a day or two between coats, then a week before applying anything over the top of it.
You can seal oiled timber with a couple of coats of shellac and then apply varnish or wax over the top for additional protection.