External authentication using Oracle Standard
I'm afraid it is practically impossible. In Oracle's terminology external authentication means: "If users's name on client machine is identical to user's username in database, then user does not have to enter ANY password". This way of authentication is very weak and must be combined with other security features, for example you allow users to login only from one trusted, secured host.

Oracle does not support any PAM modules (like Informix for example) or whatever. The only option is how to connect Oracle to an external auth system are:


Kerberos/RADIUS - requires Enterprise edition, plus Advanced security option, which might require extra licencing
N-tier authentication - something like Unix su on Oracle level. But this requires support on application side.
Password protected role enabled by Logon trigger.