Somewhere I read that OAuth is an open source alternative to Siteminder altenative.
OAuth is an open standard to authorization. OAuth provides client applications a 'secure delegated access' to server resources on behalf of a resource owner.
And OpenAM seems to be for Authentication probably for Adatptive Authentication.
OpenAM provides open source Authentication, Authorization, Entitlement and Federation software. Through OpenAM, the community actively continues development of OpenSSO.