By default, if you install Oracle and don't use the network configuration assistance you get a ORA_HOME without an SQLNET.ORA file. Without it, Oracle will not be able to use O/S authentication.
To solve this problem, simply create a file named sqlnet.ora in your ORA_HOME\network\network\admin directory and add to it the following line:
SQLNET.AUTHENTICATION_SERVICES = (NTS)
I hope it helps ........