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 sensible-editor /etc/ld.so.conf.d/oracle.conf

Update the configuration by running following command:

sudo ldconfig

Try to connect using:

sqlplus username/[email protected]//dbhost:1521/SID

or:

sqlplus testuser/password