You do know he took a fodder gurl and some random cm user bodys before right?
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As you can see when he took there bodis, he never went "bawards" nor did he get weak or lose power. He simply took there body and put his powers in them. There powers is an extra added bonus, facts from the manga!
To futher prove my point and end all doubts. Once sasuke took the weaken sick, no ninjutsu Oru body... Did he get weak???... Could he not use ninjutsu aswell???.... NO! Sasuke stayed the same and actually got stronger with the added Oru benefites from gaining his body!
Moral is: Oru is still Oru, but has all the powers of his victims added in him. Think about it like this...
After he takes sasuke body, he will still have 3 years to switch, but he will always keep sasukes Power/Sharigan!
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As you can see here, he took many random "fodders" and never got weaker.