"...coordinate...oil production policies in order to stabilise the oil market and to help oil producers [i.e., countries exporting oil] achieve a reasonable rate of return on their investments."
This is basically the definition of a cartel, but let's look at a more formal definition. From Wikipedia, we read:
"A cartel is a formal (explicit) agreement among firms.... Cartel members may agree on such matters as price fixing, total industry output, [or] market shares...The aim of such collusion is to increase individual member's profits by reducing competition."
(Notice that the definition assumes the participants are firms or companies rather than countries. Monopolies and cartels are usually associated with large firms, but we will see that the same definition can include countries as well as firms.)
Within most countries, anti-trust laws forbid companies from forming cartels or colluding to fix prices. In the U.S., this is why most high profile mergers must be approved by the Federal Trade Commission. The FTC examines market conditions to ensure that the proposed entity would not control the U.S. market.
With OPEC, however, we are not talking about companies, but rather sovereign governments that can do whatever they want. The U.S. cannot pass a law to prevent one foreign country from forming an alliance with another country. Therefore, the U.S. has no direct power over OPEC.
So on the surface, it would appear that we have little hope of preventing OPEC from keeping prices high. It turns out, however, that it is much more likely for a cartel to do a poor job at cooperating than it is for it to effectively control prices. This is because each member of the cartel has an incentive to cheat.
In the case of OPEC, it must control its production output in order to keep supplies down and prices up. To do so, OPEC has traditionally assigned a quota to each member country. Since the OPEC countries are so dependent upon oil revenues, each wants to make sure its quota is as large as possible; so they haggle over why their own country should receive a higher quota than what the other countries want them to have. Once the quotas are assigned, each has an incentive to cheat and exceed their quota. The logic goes like this: "The OPEC quota is limiting total output and maintaining the price level. My government really needs additional revenues to meet its obligations, and at these prices, I could make a lot more money if I produced just a little in excess of my quota. An extra 100,000 barrels a day won't make a difference..." This, in essence, is the thought process of each country, so they all produce "just a little" too much, and as a result, there ends up being enough supply that prices don't rise to the level they are targeting.
The foregoing describes OPEC as it has existed for most of its history -- a dysfunctional cartel that tries, but largely fails, to control prices. As a result, the world has benefitted from cheap oil for many, many years. It has generally required something more ideological than price control to unify them to the point of efficacy. The first such uniting event came in October 1973 during the Arab Oil Embargo.
The Arab countries were incensed by the West's support of Israel in the Yom-Kippur war, and announced they they would no longer ship oil to the United States, the Netherlands, and later, most other western countries. For the rest of the 1970's, OPEC countries demanded, and received addtional revenues from the oil found within their borders.
In short, the balance of power in the oil market shifted from the "Seven Sisters" to sovereign governments. Eventually many of the OPEC countries negotiated settlements with the international oil companies, and paid them to go away, gaining control of the fields. ARAMCO is a perfect example:
The California-Arabian Oil Company (predecessor company of ARAMCO) was founded in the 1930's by the predecessor companies of Chevron and Texaco. The company was granted access to explore for oil by the Saudi governement. In 1938, the company made its first discovery. In the 1940's, the predecessor to Exxon bought in as an additional investor, and the name was changed to the Arabian American Oil Company (ARAMCO). In 1950, the king of Saudi Arabia threatened to nationalize the company if it did not agree to share its profits 50/50 with the government. (The king followed the example of Venezuela where same thing happened a few years earlier.) The companies were forced to concede, and thus the contract was changed. In 1973, the Saudi government acquired 25% of ARAMCO, increased this to 60% in 1974, and by 1980, had acquired 100% of the company, forcing the international oil companies out. Today, the company is now called "Saudi ARAMCO," which is a bit ironic, considering the underlying meaning of the acronym.
The volume of discoveries in OPEC countries dropped dramatically and immediately following the "buyouts" of the international oil companies, and has never recovered. Due to excess capacity, the national oil companies (NOCs) of OPEC nations had no incentive to explore. Why explore when they already had too much?
The graph below shows worldwide disoveries over time in millions of barrels, split into volumes found in OPEC nations (darker bars on top) and non-OPEC nations (light bars on bottom). There is a dotted vertical line indicating the year 1980, the year in which Saudi Arabia completed its takeover of ARAMCO. Visually, note the abrupt change in OPEC discoveries from pre-1980 to post-1980. It would be difficult to believe that this step change was a result of natural phenomena, rather than a result of the international oil companies' loss of access to the region.