Since 11gR2, OUD can be used natively as a OAM Identity Store. Select "OUD: Oracle Unified Directory" as Store Type as described here.
As an alternate solution, you can also configure OVD as Identity Store with OAM and then configure LDAP adapter for OVD with OUD details.Configuring Identity store for OAM is documented here.
Choose "OVD: Oracle Virtual Directory" as store type and provide
store details as per the document. Configuring LDAP adapter for OVD is documented here.
Provide your OUD details required as per the document.