The main difference between Oligarchy and Plutocracy is that the Oligarchy is a power structure and Plutocracy is a society or system ruled and dominated by the small minority of the top wealthiest citizens.
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.
Plutocracy (Greek: πλοῦτος, ploutos, ‘wealth’ + κράτος, kratos, ‘rule’) or plutarchy, is a form of society defined as being ruled or controlled by a function of wealth or higher income. The first known use of the term was in 1631. Unlike systems such as democracy, capitalism, socialism or anarchism, plutocracy is not rooted in an established political philosophy. The concept of plutocracy may be advocated by the wealthy classes of a society in an indirect or surreptitious fashion, though the term itself is almost always used in a pejorative sense.
A government run by only a few, often the wealthy.
Those who make up an oligarchic government.
A state ruled by such a government.
Government by the wealthy.
A controlling class of the wealthy.