"How do I treat my cutting board?" Well, we'll tell you!
With Food Grade Mineral Oil.
That was a simple answer, wasn't it? You don't get that too frequently on the internet! But now that you have your answer, let's chat a bit, hm? Starting with the "uh-no's!"
You DON'T want to buy just any kind of mineral oil. The stuff used for babies? That's perfumed and the texture is a little off, to be frank. I love the smell of lavender, but I don't need it saturated deep into my cutting board.
You DO want a viscosity level of 70. Any Food Grade Mineral Oil (FGMO) will have this level of viscosity so you don't have to worry about it, but if you were to buy from an oil company or distributer, that would be important information.
You DON'T want to use vegetable oil. People will gasp at the thought of using a petroleum product over something natural -- and I completely understand that BUT vegetable oil goes rancid. Do you want to regularly and chronically get sick from inadvertently consuming something that goes rancid or do you want to risk the tiniest of tiniest chances that you're going to consume enough petroleum that over a few decades it builds up to cancer... maybe, very maybe? The truth is is that if you're living in a city and enjoying all it has to offer, your most immediate plan should be to move so you're not consuming so much polluted air. That has a higher likelihood of harming you than picking up trace amounts of petroleum from you wooden cutting board.
You DO want it to be "white" as it is sometimes called or more frequently called "clear." Why is it called white? Beats me.
Maybe you're not sold on mineral oil, that's fine! Because we're going to be covering alternatives soon enough :)