It may not be the cause of your error, you shouldn't set attributes to the Root class when using it as a CherryPy application since CP is threaded. Rather attach it to the current request object.
Though it might not answer your specific use case, you might want to look at this code for an example of OAuth with CherryPy:
The tool that attaches the current user to the current request:
The plugin that provides OAuth support for the application:
The app itself:
How you setup everything together: