Complex sentence: The Omnicom Media Group MENA (Middle East and North Africa), a regional holding company, was formed which reports to the Omnicom Media Group in Europe, the Middle East and Africa based in London.
Split into simpler sentences: The Omnicom Media Group MENA (Middle East and North Africa), a regional holding company, was established. It reports to the Omnicom Media Group in Europe, the Middle East and Africa based in London.