Tyrant: The power is concentrated in the hands of one person. Examples: Hitler's Germany, Stalin's Soviet Union, the word comes from the Greeks though, so ancient Greece is a good example as well. The Tyrant is the word for the ruler, not the country suffering it.
Oligarchy: The power is concentrated in the hands of a small group. It is like instead of having one tyrant, you have a handful of them. This is also considered to be a dark state of a country. Comes from ancient Greeks as well, they had times when the country was an oligarchy.
