I have no personal experience with Kerberos, but the process of configuring Oracle Net for Kerberos Authentication is explained in the "Oracle Advanced Security Administrator's Guide", and the way I understand it is that it is transparent to Oracle Net applications (such as your Delphi application). The user connects without specifying a username/password, and instead Oracle Net uses the Kerberos ticket on the client machine.
one more thing we have D7 license for DOA components. when we upgrade to xe2 or higher do we need to pay for DOA upgrade?