For First Question!
OpenID is about authentication (ie. proving who you are), OAuth is about authorization (ie. to grant access to functionality/data/etc.. without having to deal with the original authentication).
OAuth could be used in external partner sites to allow access to protected data without them having to re-authenticate a user.
So What is your use Case of having OpenID as a Mediator?
For Second Question! Yes it is possible, Wso2 has OpenID as authenticator in carbon platform, please refer to this! How to install it!
and if you wonder how to make use of SSO in carbon products Please refer to this