OpenID vs. OAuth [duplicate]

OpenID is a way to specify one identity for multiple sites so you don't need to register over and over again.

OAuth is a way to allow one application access to one account without giving said application your account login information. You can use them in conjunction.

More info: OAuth-OpenID: You’re Barking Up the Wrong Tree if you Think They’re the Same Thing

If you have an account (with some private resources) in a website, you can log in with username/password couple. If an application would like to get some private resources, and if you don't want to give them your username/password, use OAuth.

But if you want to log in into multiple websites with a unique account, use OpenID.

(Some websites use OAuth like OpenID, and OpenID can be use like OAuth if you have some private stuff in your OpenID account)

OpenID = using login credentials from an OpenID provider (Google) to login to another application (Stack Overflow)

OAuth = Allowing an application (TwitPic) to act on your behalf to and access information from an application that you use (Twitter).

They can be used in conjunction with each other.