The credentials an Oracle database will accept is based on what back-end authentication provider(s) it's configured with. If you create a DB Connect identity (username and password) and pass it to Oracle, Splunk doesn't really know or care if Oracle is looking in its local store, an LDAP directory, or somewhere else. As long as they're correct, your credentials should work fine.
On the topic of DB Connect and LDAP, if what you really need is to use an LDAP URL to connect to the database, there's information about using LDAP instead of JDBC in the database connection section of the DB Connect manual.
