Gojo Kibafuda gives Tobirama the win in scenario two.
Scenario 1 is complicated. Tobirama hasn't shown a jutsu apart from that multiple explosions that would be able to kill Oro at once(though I am positive he should have one. However theories are not facts). The only other option would be to force the overuse of Oral Rebirth and then kill the weakened Orochimaru. Tobirama's speed, once he has marked Oro,will render most of Oro's attacks pointless. Once he tires him out, he kills him with great ease.
As for the battle...Snake summons get torn apart by Suiton. Tobirama suiton mastery would take care of Oro's fuuton and other techniques. Eventually, after enough playing around with big techniques and summons, it becomes a close quarters contest and Tobirama would dominate. Oro would find it difficult to formulate a feasible strategy against someone who can not just teleport from any danger in the blink of an eye, but can also see the danger coming from a mile away due to incredible sensory skills. Kusanagi would also be useless considering Tobirama's kenjutsu mastery augmented by FTG