Because of cartels ( an element of collusion of firms) , oligopolistic firms are seen to behave like monopolists. Hence, firms in oligopoly market structure earn abnormal profits in the longrun.

The firm is in equilibrium at point E producing output OQO at a cost OC and selling it at price Opo. The firm therefore earns abnormal profits equivalent to shaded area OCPoAB because at equilibrium AC is less than AR.

The price OPo is an administered price and for that matter it is rigid i.e. does not change frequently.