Hi Bruno,
In your case, you perhaps need an Identity Management solution (rather than a SSO solution) to sync passwords among different applications, for instance SAP Identity Management (SAP IdM).
First of all you need to have an app to host all the user accounts. In your case, it should be Oracle LDAP.
Then when creating user account on Oracle LDAP or user changes its password on Oracle LDAP, this app should be able to catch this password and send it to IdM via API.
Next, IdM is able to send this password to all the systems it connects to, in your case, SAP ECC or Portal. SAP IdM is capable of provisioning productive password to SAP ABAP/Java and even Microsoft AD.
Best Regards
Chenyang Xiong