An oligopoly is formed when a few companies dominate a market. Whether by noncompetitive practices, government mandate or technological savvy, these companies take advantage of their position to increase their profitability. Companies in technology, pharmaceuticals and health insurance have become successful in establishing oligopolies in the U.S.

Computer Operating Systems

New high tech markets can become oligopolies when the companies provide unique products that are supported by an ecosystem of supporting technology. Computer operating systems in 2012 are dominated by Microsoft's Windows, Apple's Mac OS and the open source Linux operating system. These three systems capture close to 100 percent of the computer operating system market due to their established positions, according to the StatOwl website. All other software providers make programs that are compatible with these systems, further reinforcing the dominance of the major players.

Smart Phone Operating Systems

The smart phone market is similarly dominated by a handful of companies, the most powerful two being Google Android and Apple iOS. Those companies have deep relationships with the handset providers and are able to have their system pre-installed on each phone. As with computer operating systems, these relationships become self-reinforcing as they grow.

Pharmaceutical Industry

The pharmaceutical industry is becoming an oligopoly due to the staggering costs of developing and marketing new drugs and because of patents that protect new products from competitors. It can cost more than $1 billion to develop a new drug, get it approved by the Food and Drug Administration and bring it to market, according to "Forbes" magazine. With those kind of upfront costs, only a handful of companies including Pfizer, Merck and Novartis, can afford to create and sell new products. The government grants those companies extended patents on their drugs, and these patents protect drug developers from competitors for many years.

Health Insurance

Health insurance is a highly regulated industry with a number of government mandates at the state and federal level. The 2010 Patient Protection and Affordable Care Act requires insurers to accept more high risk patients as customers and to provide comprehensive coverage to all their customers. Such constraints favor a handful of established companies, such as Humana, Cigna, Aetna and WellPoint. Some observers suspect that companies capable of surviving new legal mandates will evolve into an oligopoly.