However, the Oath trigger will only do anything if you still have more creatures than your opponent when the trigger resolves, so in this case, it won't actually do anything. Oath trigger checks to make sure the condition "you have less creatures" is true twice. Yes, Oath triggers, but you won't be able to search out a creature.
I think you've got that a bit backwards. I know you know how Oath works, but here we're concerned with having
fewer creatures than your opponent, not
more. As Jebus said, the creature will return to your opponent, so all else being equal, the Oath will trigger and you'll be able to dig up a dude to put into play.