It’s not obvious to set it right, so I’m putting some notes here:

Installation is described here.
Few comments:

• ORACLE_HOME is needed just for installation
• If you add client library path to /etc/ld.so.conf.d/oracle.conf   and update ldconfig, you don’t need to export modified LD_LIBRARY_PATH
• when you install Oracle client library and set environment,  you can install cx_oracle also via pip install cx_Oracle

The crucial step not mentioned in the installation guide is  to set NLS_LANG environment variable – this should be in the environment of your python program using cx_oracle.  So for instance for Flask+SQLAlchemy you can have:

if app.config.get('SQLALCHEMY_DATABASE_URI').startswith("oracle"):
    os.environ['NLS_LANG']= 'AMERICAN_AMERICA.AL32UTF8'

Without this variable oracle client is using 7bits ASCII! So any unicode character will raise “UnicodeEncodeError: ‘ascii’ codec can’t encode character” error.