First of all you need to download Instant Client Downloads. Install alien software so you can install rpm packages by typing following command in terminal.

sudo apt-get install alien

Once that is done, go to the folder where the rpm files are located and execute the following:

sudo alien -i oracle-instantclient*-basic*.rpm
sudo alien -i oracle-instantclient*-sqlplus*.rpm
sudo alien -i oracle-instantclient*-devel*.rpm

You need to install libaio.so. Type following command to do it:

sudo apt-get install libaio1

Create Oracle configuration file:

sudo vi /etc/ld.so.conf.d/oracle.conf

Put this line in that file:

/usr/lib/oracle/<your version>/client/lib/

Note - for 64-bit installations, the path will be:

/usr/lib/oracle/<your version>/client64/lib/

Update the configuration by running following command:

sudo ldconfig

Try to connect using:

sqlplus username/password@//dbhost:1521/SID

or:

sqlplus testuser/password

Note that if you installed the 64-bit version, the client is called sqlplus64.

Source: http://askubuntu.com/questions/159939/how-to-install-sqlplus