Is it a new board or one you've had for a while? A new board will soak up more oil than an old board. To pull up something from an old thread:
NEVER use anything except mineral oil on your board - vegetable, nut and animal oils will go rancid. Oil your board once or twice a day until it stops absorbing oil - this might take a few days. Just give it a good heavy dose of oil ... and repeat until it stops absorbing the oil - then it's ready to use. My edge-grain board took about 3 days, my end-grain board took about a week.
Regarding the mineral oil - just get the stuff from the drugstore (probably labeled Mineral Oil USP). That's medical grade mineral oil. Food grade mineral oil is certified by a different government agency ... since it's intended for use in food preperation. They are, for all practical purposes, the same thing ... although food grade allows some things not allowed in medical grade - and usually costs more.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain