Difference Between Monopoly And Oligopoly

Oligopoly is a market structure containing a small number of relatively large firms that often produce slightly differentiated output and with significant barriers to entry. Monopoly is a market structure containing a single firm that produces a good with no close substitutes and with significant barriers to entry. While it might seem as though the difference between oligopoly and monopoly is clear cut, such is not always the case.
A comparison between these two market structures is bound to be illuminating.
•One or Few: The primary difference between oligopoly and monopoly is that monopoly contains a single seller, whereas oligopoly has two or more sellers. Such a difference might seem to provide a clear separation. But not necessarily.

•Substitutes: