Is there an oAuth Demo?

Yes we have a working demo that you can use. There is a version for production and staging. 

In each instance the demo authenticates you with our test app. You can revoke permission for this app at any time by visiting the permissions page. 

• oAuth Demo (Production): https://identity.justgiving.com/demo
• oAuth Demo (Staging): https://identity.staging.justgiving.com/demo 

The working demo also gives you a Javascript snippet you can use to quickly spin up a working prototype.