create account with id & password same as unix

Sorry for this trivial question. (The manual explanation is not too clear)
Can I create account for oracle (we run Ver 6.0.31 on AIX) such that the id is same as the unix user-id and the password is automatically obtained from that of the unix. In other words, the user will go into oracle without typing username and password (for sqlplus, sqlforms30 .... etc). How do I create such accounts and passwords for oracle with such.

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