Step-by-step explanation:
The main difference between monopoly and oligopoly is in the number of firms under consideration.
In Monopoly, the market witness a situation in which one company alone dominates; in other words, this company produces goods or services without having any close competitors.
While in Oligopoly, not just one but a small group of companies act as the dominant players in the market even though they may produce slightly different products; they thus influence the market a lot reducing the chances of new competitors.