Original owner the body is probably one of this 3.
Dead dead, the MC soul someway anchored to his body and returned to life, there is some miracle behind but it could never get explained
Changed spot with MC and died in his body
Is buried into the body and will give problem in some future arc trying to get back control of it, the worst possiblity for me, I can even see a cliche ending with MC doing everything he couldn't in life (training, loving,having a true family, revenge) and finally ascending to paradise, and then the OG guy wake up very confused