How to add logout feature to an OpenID enabled site?

I have recently added OpenID login to my website. But I don't know how to add the logout feature.

For example, on clicking the Sign In button I am able to show the login form of the selected OpenID provider, for example Google Account. However, I don't know how to implement a Sign Out button to sign out of the Google Account.

Please advice.

Solution

OpenID relying party can't log user out of OP, you can only implement local logout. Just like RP can not log user into OP.