The first part is to allow openID users to post comments on your blog, this can be easily achieved by installing Wordpress OpenID plugin. The installations is simplicity itself - just copy the plugin and activate it, nothing else required.

The other part is being able to use your blog as an openID server. I wasn’t able to find this as a plugin so I decided to go with a standalone
phpMyID. Grab the 0.7 beta here and follow the instructions in README file, they are pretty straightforward and they worked for me with no problems.

Just to note that if you want it to work with your wordpress blog you need to add to the header.php file of your current theme

If your blog is on a subdomain then MyID.config.php and MyID.php should be uploaded to the subdomain root(i.e. your-domain/subdomain).
Otherwise if you just put it into your root, it will point to your openID server - i.e. the root of your domain