Here’s the how to:
1) First, and most important of all—use the right type of oil. Mineral oil. Very, very important! Forget all those veggie oils—the overwhelming majority of them will turn rancid. Which can add an unwanted tinge of tartness to whatever you chop on the board and make the surface gummy and mildly disgusting.
(Here’s a brand of mineral oil I recommend: Thirteen Chefs Food Grade Mineral Oil.)
2) Apply a modicum of oil to the board, and with a clean cotton rag, spread and rub it into the wood. Repeat; repeat. (It’s better to apply the oil to the wood rather than the cloth because you’ll use less. Cloths tend to be oil hogs.) Make sure to do both sides as well as the edges, even if you only chop on one side of the board. Wood needs to absorb liquids evenly—otherwise it tends to warp.
3) Don’t over oil! Better to do a second session if need be. (If you ignore this advice, you’re going to keep wondering where all those oil smears on your countertop and clothes are coming from.)
4) After you’ve covered the entire surface, let it sit for a minute or two. Then, using a fresh dry cloth, wipe the entire board taking away any excess oil. Prop the board up with nothing touching it, so it can get as much air as possible. Allow the wood to do its absorption thing. My favorite airing-out spot is on top of the stove grills. (I do this after dinner and usually flip the board at least once.)
5) If after 4 or 5 hours the board is still moist with oil, you’ve applied too much. Take a clean cloth and rub it down again and continue to leave it out to dry. All you can do is wait—eventually the wood will soak it up. (And if you need to use the board in the meantime, go ahead. Mineral oil is virtually tasteless and won’t hurt your food.)
Enjoy the mellow shiny glow of an oil-freshened board! For more details on the world of wooden cutting boards, including cleaning them and how they compare to plastic, please see my in-depth articles Cutting Boards — What’s Better, Wood or Plastic? and Cutting Board Cleanliness. . . on the KitchenKnifeGuru website.
P.S. If your wooden cutting board had begun to develop some minor cracking, this should help—but it can’t do miracles. It can also help with preventing warpage.