This extension lets users log in with an OpenID instead of a username
and password. An OpenID is a special URL that people can use to log in
to a Web site. The extension also lets users who have an account on
the wiki log in to other OpenID-aware Web sites with their wiki user
page as their OpenID.

You must create a table in your MediaWiki database to hold the OpenID
URL mappings. The openid_table.sql script in
/usr/share/mediawiki/extensions/OpenID/ should do the
trick. Typically you do this using the mysql command-line client, like
so:

mysql -h yourdbhost -u youradminuser -p yourwikidb < openid_table.sql

In your MediaWiki LocalSettings.php, add the following line some place
towards the bottom of the file:

require_once("$IP/extensions/OpenID/OpenID.php");

Theoretically it should work out of the box, but you'll almost
definitely want to set the trust root and access controls.