The main difference between Aristocracy and Oligarchy is that the Aristocracy is a form of government in which power is in the hands of a small, privileged, ruling class and Oligarchy is a power structure.
Aristocracy (Greek ἀριστοκρατία aristokratía, from ἄριστος aristos ‘excellent’, and κράτος, kratos ‘rule’) is a form of government that places strength in the hands of a small, privileged ruling class. The term derives from the Greek aristokratia, meaning ‘rule of the best-born’.The term is synonymous with hereditary government, and hereditary succession is its primary philosophy, after which the hereditary monarch appoints officers as they see fit. At the time of the word’s origins in ancient Greece, the Greeks conceived it as rule by the best qualified citizens—and often contrasted it favourably with monarchy, rule by an individual. In later times, aristocracy was usually seen as rule by a privileged group, the aristocratic class, and has since been contrasted with democracy. The idea of hybrid forms which have aspects of both aristocracy and democracy are in use in the parliament form.
Oligarchy (from Greek ὀλιγαρχία (oligarkhía); from ὀλίγος (olígos), meaning ‘few’, and ἄρχω (arkho), meaning ‘to rule or to command’) is a form of power structure in which power rests with a small number of people. These people might be distinguished by nobility, wealth, family ties, education or corporate, religious or military control. Such states are often controlled by families who typically pass their influence from one generation to the next, but inheritance is not a necessary condition for the application of this term.
Throughout history, oligarchies have often been tyrannical, relying on public obedience or oppression to exist. Aristotle pioneered the use of the term as a synonym for rule by the rich, for which another term commonly used today is plutocracy.
Especially during the fourth century BCE, after the restoration of democracy from oligarchical coups, the Athenians used the drawing of lots for selecting government officers to counteract what the Athenians saw as a tendency toward oligarchy in government if a professional governing class were allowed to use their skills for their own benefit. They drew lots from large groups of adult volunteers to pick civil servants performing judicial, executive, and administrative functions (archai, boulē, and hēliastai). They even used lots for posts, such as judges and jurors in the political courts (nomothetai), which had the power to overrule the Assembly.
The nobility, or the hereditary ruling class.
Government by such a class, or a state with such a government
A class of people considered (not normally universally) superior to others
A government run by only a few, often the wealthy.
Those who make up an oligarchic government.
A state ruled by such a government.
the highest class in certain societies, typically comprising people of noble birth holding hereditary titles and offices
“members of the aristocracy”
a form of government in which power is held by the nobility.
a state in which governing power is held by the nobility.
a group regarded as privileged or superior in a particular sphere
“Britain’s pop aristocracy”
“a new aristocracy of talented young people”