Connecting to Oracle using SQL*Plus without Tnsnames
You know where Oracle runs, you know the user and password as well as the database name. You could try to locate the tnsnames file or create one on your own. If you don't like that, simply use the command shown below.
Recipe #1 - Connecting to Oracle using SQL*Plus
sqlplus [email protected]:port/database
A complete example which connects to the database called XE on the server 192.168.161.240 using the user scott with the password tiger:
sqlplus [email protected]:1521/XE