Oligarchy is from the greek ολίγοι ("Oligoi") + (άρχς) archy = rule/ruler --> Oligarchy; rule by the few.
One definition says that oligarchy is a debased form of Aristocracy. But that is not actually usually true because most aristocracy is not as depicted in myth but the product of local oligarchs with the power to pass on their Kakistocracy (rule by the worst) to their children. Oligarchy is a product of hierarchy and one step removed from dictatorship. To have a tyrant he/she needs a hierarchy of power beneath him or her. And one can have tyranny without a dictator in the form of oligarchy. Oligarchy is unstable because the "few" rarely can sustain their divisions of property and power, and so wind up fighting each other, even as they tend to gang up on the many. A Monarchy is an oligarchy with one person at the top of it's pyramid. A dictatorship is a monarchy without the guarantee of an orderly transition of power from ruler to ruler or stable rule of law.
Oligarchy often resembles democracy as the oligarchs setup republican forms and representation among themselves. Moreover, oligarchy can emerge from a Republic only as oligarchs are able to amass followers. Oligarchy can be a feature of a government in it's subdivisions and sub-functions. When grifters can acquire control, ownership or access to vital resources, "monopoly" through the various channels of privateering they become either oligarchs or have tyrannical monopoly over that resource. Eventually they turn that power, economic and/or political, into control over other resources. When that happens a society starts to have the features of Oligarchy. Typically, eventually the oligarchs dispense with all but the forms of republicanism. When that happens a property becomes an oligarchy or even a dictatorship.