Is there a way to connect to an authentication webservice or a BAPI to SAP?

I have a mobile application but I want to use sap users iso outsystems users (will have to use OS users for ease of use but that will be in the background) I need something like a login BAPI or a login webservice, or is there some kind of oAuth option ? I want to replace the login page with a login to sap .. 

I can call the BAPI's but with a single user (and that will be included in code)

To specify the user calling a web service you can use action SetWebReferenceCredencials.

To SAP Connector you specify User/Password, but you can also use Dynamic Login for each SAP Connection.

I guess SAP has some Login functions. Search in the available RFC/functions.