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Monopolistic and Oligopolistic are pretty much the same except for the size of the manufacturing companies. In simple terms, a monopoly is where there is a single seller in the marketing industry and an oligopoly is where there is small group of sellers in the same field in marketing industries.
When it comes to a monopolistic competition, this is where a market structure has a large number of sellers, each of which is relatively small and posse a very small market share. An oligopoly market is where there are fewer large producers who are present in the industry world and account for most of the output in the industry, there are many small firms but these few large firms dominate and have concentrated market shares. Oligopoly also has more barriers to entry than a monopolistic.
I would say that Comcast the cable company is a Monopolistic competition in the market right now. The reason I say this, is because there is not much options for entertainment when it comes to cable. Comcast has made their name around the country, providing what other companies are not. Making them a monopoly amounts other companies.
I would consider breakfast cereal companies an oligopoly because there are four breakfast cereal manufactures that I know of, Kellogg, Post, Quaker and General Mills. Because there are only four companies competing to make breakfast cereal, they are called an oligopoly. Whereas, only one company competing to provide cable TV to the customers, they are called monopoly.