A comparison between these two market structures is offers a little insight into each.

• Many or Few: The primary difference between oligopoly and monopolistic competition is the relative size and the market control of each firm based on the number of competitors in the market. However, there is no clear-cut dividing line between these two market structures. It is not possible to say that some number like 50 firms is the dividing line, such that 50 firms make it an oligopoly and 51 make it monopolistic competition.
  
  While one industry containing only 3 firms is clearly oligopoly and another industry with 30,000 firms is undoubtedly monopolistic competition, the structure of an industry with 30 firms or 300 firms is not as obvious.
  
  Such industries might have characteristics of both oligopoly and monopolistic competition. As the number of firms decreases, the firms tend to behave more like oligopoly and less like monopolistic competition.


• Dominance by a Few: In some cases status depends more on the dominance of a few firms rather than the total number of firms in the industry. An industry with 3,000 relatively equal firms is most assuredly monopolistic competition. However, an industry with 3,000 firms, that is dominated by 3 relatively large firms, is most likely oligopoly. For example, the petroleum extraction industry has thousands of firms, but a handful of the largest firms dominate the market, making it an oligopoly.


• Geographic Area: In other cases, the geographic size of the market is the prime determinant of market structure. A particular industry might be monopolistic competition in a large city, but oligopoly in a smaller town. Retail sales offer an example. Larger cities usually have hundreds, even thousands, of shopping alternatives, including discount super centers, mom-and-pop stores, shopping malls, and nation-wide chains. Such a market is monopolistic competition. Smaller towns, however, tend to have fewer stores, perhaps a single super center or shopping mall and a handful of stores located in a small downtown area. Such a market is oligopoly.


• Barriers to Entry: A key difference between oligopoly and monopolistic competition is barriers to entry. Oligopoly barriers are high. Monopolistic competition barriers are low. However, entry barriers are a matter of degree. The need for government authorization is one entry barrier that can create oligopoly, especially if entry is limited to only a few firms. However, it can also create monopolistic competition if a larger number are allowed entry. Other barriers, such as start-up cost and resource ownership, also limit entry to different degrees, leading to either oligopoly or monopolistic competition. Moreover, these entry barriers can change over time, transforming oligopoly into monopolistic competition or vice versa.