Is there any node.js module that I can use for paypal oauth authentication?
None of the Node.js auth solutions support PayPal currently, as far as I'm aware of. I'm the author of Passport, and implementing a PayPal auth strategy is on my to-do list. If you want to get started before I get around to it, let me know and I'll help out.
I don't know of any modules specific to PayPal, but if you end up deciding to roll your own, since you're using Express/Connect I would recommend looking into using EveryAuth and just adding a PayPal module to it: https://github.com/bnoguchi/everyauth -- that'll make integrating with any additional authentication APIs much easier in your app's future.