How to use OpenLDAP for Oracle database user authentication
Permitted Features, Options, and Management Packs by Oracle Database Offering


  Enterprise User Security is a feature for directory-based management
  of database users. It requires a corresponding Oracle Identity
  Management Directory Services Plus to be licensed.


Enterprise User Security: The Big Picture


  Enterprise User Security requires Oracle Internet Directory 10g
  (9.0.4) or higher. Other LDAP-compliant directory services are
  supported by using Oracle Internet Directory Integration Platform to
  synchronize them with Oracle Internet Directory. Another directory
  services product, Oracle Virtual Directory, provides a single, dynamic
  access point to multiple data sources through LDAP or XML protocols.
  Oracle Virtual Directory can provide multiple application-specific
  views of identity data stored in, for example, Oracle Internet
  Directory, Microsoft Active Directory and Sun Java Systems Directory
  instances, and can also be used to secure data access to the
  application-specific sources and enhance high-availability to existing
  data-sources.



Through Oracle Virtual Directory or Oracle Unified Directory.
See above. Technically it should be possible to use as standalone, but not supported at all.
None for direct connection. See above.