I was thinking Orochimaru has the perfect condition to be the 5ths SO6P after rikudo,madara,nagato and obito.
lets see the background of few SO6P
1.Madara stole Senju DNA so he awakned rinnegan in his old age
2.Obito Implanted Madara's rinnegan in Nagato so the implantation of eye made him 3rd...