Be sure to scroll down... there may be more than one question on this page!
Oak is not an ideal material for a moist area. It tends to blacken over time when in contact with water. However, this is an aesthetic and not a functional problem. But that's why you don't see oak butcher block very often. Also, oak's open grain will be more difficult to keep clean compared to, for example, the tighter grain of maple.
Since this is a food surface, you have limited options for staining and sealing. I would not use a stain because common wood stain is a toxic substance, and not approved for food surfaces. I am unaware of any wood stain that is approved for this purpose.
As far as sealing the surface, you can take the primitive path and use mineral oil to seal it. You can't use a vegetable oil because it will become rancid.
However, the more professional alternative is to use a sealer designed for food surfaces. Both Behlen and General Finishes make products called Salad Bowl Finish. I'm sure there are other companies who produce similar products. This product is approved for food contact on countertops, cutting boards, and... yes... salad bowls.
Though staining is not recommended, you could try using a wood bleach such as oxalic acid to lighten the oak before sealing. I would get a scrap of the same oak and experiment before committing to do this on an expensive butcher block top.
Oh... and get a check up front!