What's the deal with Orochimaru and seals? During the chunin exams, when Orochimaru attacked Konoha, the Third Hokage managed to seal the hands of Orochimaru, rendering him incapable of using any jutsus. 

But later in the series we see that Orochimaru did use jutsus. For eg, during the battle with Naruto he uses the Kawarimi no Jutsu. 

Though it has not been depicted that he uses hand seals to perform Kawarimi no Jutsu, but I presume that a jutsu of that level needs a hand seal to be pulled off. 
So, Orochimaru had never lost his jutsus. 

Next, Sasuke managed to absorb Orochimaru into his body which was sealed off by Itachi using the sealing powers of Sword of Totsuka. How then is it possible for Orochimaru to be present in someone else's body and "pop out at will". 

So how is it that no sealing techniques work upon Orochimaru?

How Orochimaru resurrected the 4 Hokages.
There was only a part of Orochimaru that was in Sasuke's body. I believe that the curse mark transfers a part of his chakra and thus a part of himself into the body of the target. 

It was the part of Orochimaru that was inside Sasuke that was sealed away by Itachi using the sealing powers of Sword of Totsuka. But a part of him still lived inside the others who carried the curse mark and his chakra was there with Kabuto, who absorbed Orochimaru after Sasuke killed his main body. 

Remember that Orochimaru was very weak when Sasuke revived him that he had to absorb his chakra from Kabuto to become somewhat normal again.



And it is not true that Orochimaru couldn't be sealed.The 3rd Hokage couldn't seal him just because he was too weak by then to pull Orochimaru with him. And it is not that Orochimaru can pop-out at will from anyone's body. Sasuke resurrected him. He didn't pop-out at will.