An oligopoly is characterized by a small number of sellers who dominate an entire market. All of the firms who partake in an oligopoly are considered to be very large in terms of profit, size and client base.
Monopolistic Competition is characterized as a form of imperfect competition, which exist when there are many sellers of a good or service but the products do not contain noticeable differences. There are several forms of imperfect competition, of which Monopolistic Competition is one.