The finish is the least of your problems -- you can't do what you're thinking of doing if you want it to hold together. Laminating a layer of solid wood, which expands and contracts with changes in moisture content, to an engineered substrate, which is dimensionally stable, is a recipe for buckling and delamination. You could use veneer, typically 1/16" or less thick, but OSB is a lousy veneer substrate. A wooden construction that looks like this is possible, but I doubt it will be so easy and cheap as you imagine.

Bob, as long as you are attaching the Ipe to the OSB with a few screws only, I don't see where this would be a problem. Jon is right that trying to laminate the two could cause problems.

However, I really don't see the benefit of using the OSB as a substrate in the first place. Since this top will only be seen from above, I would use pocket screws on the underside to reinforce the edge-glue-up of the boards. Additionally, I would be sure to wipe down the ipe with acetone before gluing up the panel, and I would use epoxy for the glue-up since this will be in a wet area. This should give you plenty of strength to the joint, so as long as the ipe is strong enough for the application, there is little need for a substrate. If you do need a substrate, I would go with 3/4 CDX ply instead of OSB, just because OSB is not good in a wet environment.

For a finish, I would probably use either a two part epoxy for its water-repelling properties or Behlen's Rockhard.