Good debate there by Kidd and Username. I would just like to point out that huge summons such as Manda would have been impractical in the fight Oro-sama VS. the Geezer because of the restriction of movements enforced by the confinement of the barrier. Probably that is the reason Oro-sama started the battle with a different kind of summon. And if I have watched the same anime as Kidd, I would say Oro-sama was clearly at a disadvantage when it came to close combat regardless of what jutsus (tai/nin) he was using because the professor seemed to have counter for everything Oro-sama was throwing. Oro-sama was definitely enjoying the third's ordeal from a front row seat as the old man struggled against those hokages, but anytime Oro-sama went for a little one-on-one against the geezer, Oro-sama (hard to admit
) was spanked. I am not even sure if the third would have used the jutsu to summon the death-god if not for those seemingly "immortal" hokages. Recall those hokages body parts were regenerating after they took direct blast from the tricky old fool, instead of being dead. Then again those two were already dead and how do you kill something that is already dead?
We really don't know how the outcome of the fight would have been different if the fight was just between Oro-sama and the Geezer. We can only make assumption. Nontheless, its a fact Oro-sama established a clear advantage when the two dead hokages were fighting with him. I don't think a summon such as manda could have done any better than those two deadly summons. The battle was fought the way it was because Oro-sama and the Third new each other very well.