Here's a theory loosely thrown together that has some valid points. On my phone so i cant post images.
1. Madara was the first person to experiment with Hashirama dna right. He created zetsu's and prolonged his life. Who else was seeking immortality and and created life from Hashirama dna? Orochimaru. He created Yamato by infusing a newborn child with Hashirama's dna.
2. The edo tensie is a jutsu used by few shinobi. But only one was shown to counter the seal. Madara. Who could have taught him the release seals? The person he gave Hashi dna to. Orochimaru. This could be the reason he knew so much about the edo tensie.
3. When Kabuto and Madara met kabuto introduced himself as "HIS" assistant. Why would he be talking about Obito if they were partners. Kabuto has only been known as Orochimaru's assistant nobody else.
4. When Sasuke was talking to Jugo and Suigetsu he talked about Orochimaru. Then an image of Anko and Madara holding his neck appeared. We know Oro put his curse mark on the necks. Anko had hers shown whose to say he didnt put one on Madara.
5. Orochimaru has experimented on Madara's body alongside Kabuto. But Madara died in a cave where only Zetsu's and Obito knew the location of right wrong. Madara has proven to be able to leave the cave to give his eyes to Nagato so maybe he could have showed Orochimaru the way in.
Just food for thought.