Secret agreements occur when companies decide consciously decide to ruin competition with collusion with competitors where cooperation is negotiated by price or market sharing so that each company has a monopoly in one part of the market. Agreements which ruin market competition can be public or secret (e.g. cartels). These agreements may be in writing (either as an "agreement between undertakings" or as part of a decision or rule of professional associations) or may be in a less formal form. Oil is the most important source of energy in the global market, and the change in its prices depends on market opportunities, affecting all the prices of the world. Oil, with its fluctuating prices, affects the gloal world strategies, and its value is growing with the development of industry, and it is linked to the quantities of global reserves. According to that purpose, the OPEC (the Organization of Petroleum Exporting Countries) legal cartel is formed, as an an international oil organization that has created by the world's brightest oil producers. By devising ways and means to stabilize prices on the international markets with cope to reduce harmful and unnecessary price movements, these countries have formed legal oil cartel that dictate world oil prices.