yeah its obviously not Oro , since Madara can most likely beat the hell out of Oro easily so i doubt Madara would be so stunned to see Oro's punk ass , no doubt the so6p or a descendant.
I was going to answer like this, but no need for that anymore. Your theory don't makes sense by looking at this facts.Orochimaru was sealed into Susanoo's sword. And hasn't been killed he cannot be summoned, and I doubt that Madara would fear him.
The first Hokage's soul was sealed by the third in their fight when orochimaru attacked Konoha.
The fourth hokage is dead, but sealed his soul with the Dead Demon Soul and cannot be summoned.