Vani,
You can create a OS login name just like you create a normal user in OS.You should create a oracle user with the same name prefixed by some string ,say,OPS$ .e.g. If you create a OS user called "Vani",you should create a oracle user called "OPS$vani" and also mention that the user is identified externally during user creation in oracle. You should set the parameter OS_AUTHENT_PREFIX in init.or to OPS$.Next time you login as vani,oracle concatenates the value mentioned in OS_AUTHENT_PREFIX with your login and validates it in the database.

Hope this clarifies your doubt.

Rajesh.