Food grade mineral oil is the cheapest stuff. The wood looks stabilized and sealed so it shouldn't bother it, but maybe think about some kind of wax on the handle to help seal between the slabs and the tang.
Looks like a sweet knife. Custom made?
If it's something like 1080 or 1090 carbon steel it will develop a patina over time and this will also prevent rust. Otherwise, just keep it dry and clean. If you have to take it out in the field in a real humid place or where it might get wet, keep a thin layer of oil on it so the rust doesn't creep up on you.
Other than that, just try not to store it in the sheath since the leather will retain moisture. If you want to polish it to a real shine, use some "flitz" or similar rubbing compound, and then keep it oiled whenever you use it and put it up to keep its luster. Personally I like to actually put a patina on mine by cutting up some potatoes or apples or something like that.