sqlplus /

how to setup so by simply input
sqlplus /
it will let me in without provider username and password?

You have to setup OS authentication to login with no username and password.

If you use UNIX, you have to create unix user and oracle user with the same name (LARRY). After that, when you connect to unix as LARRY, you may use:
sqlplus /

CREATE USER "LARRY" IDENTIFIED EXTERNALLY DEFAULT TABLESPACE "USERS" TEMPORARY TABLESPACE "TEMP" PROFILE DEFAULT ACCOUNT UNLOCK;

Hi there,
I would avoid external authentification:- As far as I know it may be a security risk. Lets say someone knows that I have a database account ops$BOB. All they need to do then is to set up an account on their NT laptop of BOB and if they can then get onto the network then they can then log into my account on the database. I know when I worked at the hospital, we had to change from using external authentification across the board for this or a similar reason (it was a good while back now)