Something I thought about regarding Orochimaru being immune to Sasori's poison.
A) Orochimaru knows how to combat poison, he granted Sasuke an immunity to it:
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Adding to that, the White Snake's blood is poison. Orochimaru knows all about the stuff.
B) Orochimaru was partnered with Sasori in Akatsuki
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Apparently they did a great deal together. Orochimaru even knew his true form. It'd be absurd to think Orochimaru, being Orochimaru, never once took samples of Sasori's poison with or without Sasori's knowledge.
C) Kabuto being a direct underling of Sasori and a double agent against Akatsuki for years upon years, it's not impossible that he couldn't have gained a sample of Sasori's poison either. He could've easily engineered a charade where he and Sasori had to kill an enemy together, thereby accessing Sasori's poison.
D) Orochimaru planned on killing Sasori at Tenchi Bridge.
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He was prepared to kill Sasori... But he wouldn't have an immunity to the most basic yet deadly part of Sasori's arsenal?
It was never explicitly stated he had immunity to it of course, but it'd be kinda ridiculous to say he didn't.