Why in a Oligopoly market "The price and the quality decisions of a particular firm are dependent on the price and the quality decisions of the rival (other) firms " ???

In an Oligopoly market, ​the price and the quality decisions of a particular firm are dependent on the price and the quality decisions of the rival (other) firms. This is because of the fact that in this form of market, very few big firms (giants) own the major control over the whole market by producing significant portion of market demand. These firms are mutually dependent on each other yet there exists a very high degree of competition among them.