OpenID, OpenSSO and OAuth

My understanding of OpenID is that it provides a way to have one site contain all your identity & peripheral info, but to let other OpenID-compliant (and user-trusted) sites re-use that info for identifying and authenticating the user. Essentially it minimizes the number of logins credentials (usernames & passwords) a user has for the internet.

My understanding of OpenSSO is that it allows you to sign-in to one site and automatically log-in to all other sites that the first site trusts. Essentially it minimizes the number of times a user has to log in to these different sites.

My understanding of OAuth is that it allows users to grant 3rd party sites certain access to their information located at one particular site. Essentially, like OpenSSO, it minimizes the number of times a user has to log in to these different sites. The different with OpenSSO is that OpenSSO logs the user into all the participating sites at once (with full privileges turned on), whereas OAuth grants finer-grained access to these participating sites.

So, first off, if anything I have said is incorrect, please begin by correcting me!

Assuming I am more or less correct, then I have the following questions/need clarification on the following items:

When would I choose OpenSSO over OAuth - just when I want to restrict access that the other participating sites have when a user logs in to one of them?
Are their different security risks for each of these technologies that I will have to consider and integrate into my app - or are they considered secure in and of themselves (basically can I rest assured that if my app uses them that my app is not open to any new attacks)?

Since these technologies are so closely related its hard for me to see the whole "forest through the trees" here - thanks in advance!

Solution

Not really the right comparison to be making. OpenID & OAuth are protocols, OpenSSO (now OpenAM) is an implementation of those and other protocols (SAML, OAuth, etc)

Generally speaking, the protocols for OpenID and OAuth are similar even though they originated with different use cases in mind. Today there is a lot of convergence around OAuth 2.0 for both federation (authentication) and authorization cases. The next generation of OpenID called OpenID Connect is built on top of OAuth 2.0 and precursors to this are already in place at Google, Facebook, Twitter, etc...

As for security, there are always some risks, particularly with implementation errors. Pick a good implementation and read the specs so you understand what the risks and countermeasures are.