theLastGnu's answer is correct, the explanation is a little bit off. The cards going to the graveyard and the battlefield do happen simultaneously, but there's more to it than all being part of Oath's triggered effect. It's because they're all part of the same action in the instructions. There's only one instance of "put" in the sentence. Compare this to Gamekeeper, in which there's 2 separate instructions, and therefore 2 separate actions, and they happen sequentially. As such, with Gamekeeper, if Dragon Breath is revealed, it will not be put into the graveyard until after the creature is already on the battlefield, so won't trigger.
It will trigger and return to play (as long as it's above the creature in your library, obviously), because by the time the creature enters the battlefield, Dragon Breath would already be in the graveyard and thus will trigger. The confusion arises from the current oracle wording saying "that player puts that card onto the battlefield and all other cards revealed this way into his or her graveyard." This, however, happens simultaneously as it is all part of Oath's triggered effect, so by the time Dragon Breath could "see" a creature entering the battlefield, it will already be in the yard.
Oath of Druids {1G} |Enchantment| At the beginning of each player's upkeep, that player chooses target player who controls more creatures than he or she does and is his or her opponent. The first player may reveal cards from his or her library until he or she reveals a creature card. If he or she does, that player puts that card onto the battlefield and all other cards revealed this way into his or her graveyard.
Gamekeeper {3G} |Creature -- Elf| 2/2. When Gamekeeper is put into a graveyard from the battlefield, you may exile it. If you do, reveal cards from the top of your library until you reveal a creature card. Put that card onto the battlefield and put all other cards revealed this way into your graveyard.
