In a representative democracy not everyone is a member of the government. In fact relatively few people are. So how is representative democracy different from an oligarchy?

Representative Democracy is different from an Oligarcy because Oligarcy is a ruling system in which only a few number of privileged people get the power over ruling and decision-making in a political system and Representative Democracy, is a political system where general public gets the opportunity to select the suitable candidates for the power.   Hope this helped!