OpenID 1.0 was not very useful and 2.0 is supposed to allow you to sign in using gmail, yahoo, etc, but the OpenID libraries (for PHP at least) are buggy in this support.
Without OAuth before i think all you get after authenticating is their OpenID identifier, which isn't much info. The user would still have to provide common profile data like name and email.
So OpenID is really just meant to allow you to have one account to login to all your sites, rather than to make it easier to sign up for new sites.
OpenID 1.0 and OpenID 2.0 both support a thing called "simple registration" (this is before the OAuth work) which lets the site you are logging in to ask the OpenID provider for some basic profile information - email address, nickname, postal code etc - which can then be used to pre-fill the signup form on the site. That's the feature that's meant to make it easier to sign up for new sites, and it's been working for several years now.
The last time I tried implementing OpenID I remembered that this was not a consistent feature in that you cannot guarantee this information from the registrant.
Without OAuth before i think all you get after authenticating is their OpenID identifier, which isn't much info. The user would still have to provide common profile data like name and email.
So OpenID is really just meant to allow you to have one account to login to all your sites, rather than to make it easier to sign up for new sites.
And really is this even a good idea? I dunno.