oligarch

As nouns the difference between billionaire and oligarch

is that billionaire is somebody whose wealth is greater than one billion (10⁹) dollars, or other currency while oligarch is a member of an oligarchy, someone who is part of a small group that runs a country.

As nouns the difference between oligarch and totalitarian

is that oligarch is a member of an oligarchy, someone who is part of a small group that runs a country while totalitarian is an advocate of totalitarianism.

As an adjective totalitarian is

a system of government where the people have virtually no authority and the state wields absolute control of every aspect of the country, socially, financially and politically.