Is it easy to oppress a population in a democracy?

Not if it still is a fully functional democracy with - very importantly - recourse to an independent judiciary. But there are - and have been - several countries that technically remained democracies, but where the leader of a party that had an overwhelming majority used this position to practically rule as a dictator and oppress his political adversaries. Turkey's President Erdogan and Zimbabwe's President Mugabe today are cases in point.