How was Orochimaru able to jump bodies?
Despite the Reaper Death Seal immobilizing Orochimaru's arms, he could still perform justu that did not require hand signs.

[...] Though the act undid most of the damage caused by the Dead Demon Consuming Seal, it still left Orochimaru incapable of using techniques requiring hand seals.

Orochimaru was able to jump bodies because the justu required does not require handle signs. How it is performed is not shown until much later in the Shippuden series, which is detailed below with a spoiler

 To perform this technique, Orochimaru reveals his true form: a gigantic white snake composed of smaller white snakes, by abandoning his current host's body. Orochimaru then swallows the new host and takes him or her to a separate, mental plane where he envelops the mind of his victim. The snake body then dies and falls apart, and Orochimaru's soul dominates the body
 Living Corpse Reincarnation