This is a little complicated.
First there is the "Google OpenID URL" or https://www.google.com/accounts/o8/id. This is useful when signing up or logging in as it will redirect to the proper URL.
You can also use your Google+ profile URL. Go to https://plus.google.com and click "profile". You will get a URL like
https://plus.google.com/112323959773786487818/posts. Remove the end to get
https://plus.google.com/112323959773786487818 and you can use this URL (I think this is the new recommended way now).
However, these all redirect to your true Google OpenID URL. The URL is your Google Profile address. This is your Google+ user ID after
https://profiles.google.com. For example, my OpenID URL is
https://profiles.google.com/112323959773786487818. This URL is useful when you are inputting your OpenID URL into an application without logging in.