" To gain control over the war "..?

- that's something Kabuto wants to do. Kabuto also said: with the powers of the sage of six paths + " that jutsu ", I will be invinsible.

So far, Tobi's biggest powers are the White- Zetzu's Army.. I think that Orochimaru knew about White- Zetzu and that it was produced by Hashirama's cells. After Oro's death, Kabuto seems to knew about So6p ( maybe he knew it all along, just like Oro did), so if they knew about him.. then Oro somehow knew that the combination Senju + Uchiha = So6p.

Orochimaru made ton's of experiments of Hashirama's cells, and by knowing that white-zetzu is made of that + knowning that he would be the backbone of Akatsuki's plans: Oro probably created a forbidden jutsu which gains control over all Zetzu's.
This way, Kabuto thinks that Tobi can't do shit against him.. if he has " that " jutsu. This way, you're one big step ahead.. of gaining control over the war and fuckin Tobi's plan big time.

Orochimaru knew or anticipated, that Akatsuki would create a 4th ninja war. This way, Oroçhimaru would benefit from that war; the Nations vs. Akatsuki -> waiting for the right time to strike -> controlling all white zetzu -> using them for attack OR draining all hashirama's cells out of them for somekind of creation.