I thought that when Hiruzen used his justsu on edo Hashi and edo Tobirama that they would be sealed away forever. How can Oro just reanimate them again?
Read the new chapter again please. Anyway to clear it up, Oro got a mask from the Uzumaki Clan Temple (Uzumaki created Reaper Death Seal) for Oro to wear on his face where they need to go to a safe place or where they were buried, and Oro cuts the stomach of Reaper Death Seal, releasing the souls after Orochimaru summons it which is done via wearing the mask. You're welcome
The scroll and the whole point of this new chapter is the explaination. Orochimaru used some Uzumaki mask to call out the Shinigami..then by having his stomach cut open (both Orochimaru's and Shinigami's) he was allowed to release the Kage and his arms.