Recently, the question came up on how to expose authentication and authorization as identity services inside of the Access Management Suite.
OES - Web Services SM configured with OAM SSPI Connector
OAM - Access Management - ASDK - with custom web services wrapper
Both definitely good choices. A few quick thoughts on when each would be appropriate.
OES WS SM can be configured with OAM SSPI connector to consume OAM sessions - ObSSO cookies, but it can also be configured to consume SAML Assertions. It can be configured to return, in response to an assertIdentity call different types of tokens, including a SAML assertion. On the authorization front, OES does fine-grained authorization. The Web Services wrapping the Java API is what it is - works well, but is not integrated with any container. It also exposes a standard authorization service with XACML.
OAM with custom web services wrapper works with ObSSO cookies and focuses on coarse grained authorization, like what is typically required for URLs. The Web Services implementation, since its built on-top of the container is integrated. Also, its very simple to take a POJO and turn it into a fully functioning web-service.
So, basically if you need to extend OAM to have a SOAP interface for authentication and coarse-grained authorization, wrapping the ASDK in a web-service seems like the way to go. If you need more of a heterogeneous implementation which is expected to integrate using standards like SAML and XACML and has fine-grained authorization requirements, then the OES WS SM solution makes the most sense.