Makes sense to carry less oxygen because until it is burned, it is nothing but dead weight.
Why not takeoff with the hydrogen tank partially filled and the oxygen tank empty, and then fill both tanks via aerial refueling. That should allow for much greater payload to orbit. It probably would require some new tanker technology to handle the low temperature liquids. Even just filling the LOX tank and not the much colder liquid hydrogen tank (should that prove to be too difficult) in the air would give you much better mass to orbit performance.