Oathkeeper doesnt buff Levi. Same damage regardless of charge.
-
Could've sworn it did, but only the damage of the arrow and not the explosion. So really it's not worth using like that. If you're going to use Oathkeeper with any bow, your best bet would probably be Wish ender or Mercury A, just make sure you don't have kinetic tremors because that will immediately consume the charge on the gloves.