Oracle deny connection to users who has no CREATE SESSION privilege. Try the following tutorial exercise, you will find out how Oracle denies connection:

>.insqlplus /nolog

SQL> connect SYSTEM/globalguideline

SQL> CREATE USER DEV IDENTIFIED BY developer ACCOUNT UNLOCK;

User created.

SQL> disconnect

SQL> CONNECT DEV/developer

ORA-01045: user DEV lacks CREATE SESSION privilege;

logon denied

Oracle error message is pretty clear.