There is always an oligarchy, whether or not it is explicit. Where there is no recognized aristocracy – where, i.e., there is a liberal political order – oligarchic society inevitably devolves sooner or later to anarcho-tyranny.
Liberalism is founded upon two fundamental principles: Equality and Liberty. The former implies the latter. If all men are equal, then no man is suited either by nature or historical happenstance, nor therefore is he legitimately entitled, to exercise authoritative sway over his fellows, so as at all to constrain their liberty. If all men are equal simpliciter, then all men are free equally and perfectly, and none may rightly rule.
But of course, Liberty and Equality are contradictory. You can’t have both. The only way to let men be free is to let them be unequal. The only way to make men equal then is to eliminate their liberty, so as to make them all the same. To make all men equally free – which is to say, equally powerful – is to institute anarchy; and the only way to do it is by tyranny. Thus, our current state: anarcho-tyranny.