OPEC is the the Organisation of the Petroleum Exporting Countries was set up in 1960. It has 12 member countries that are known as a cartel when it comes to oil production. These countries are:-

Algeria, Angola, Ecuador, Iran, Iraq, Kuwait, Libya, Nigeria, Qatar, Saudi Arabia, United Arab Emirates & Venezuela

The daily oil production in barrels per day:-

Algeria                  1,667,000
Angola                  1,784,000
Ecuador                505,000
Iran                        3,680,000
Iraq                        3,115,000
Kuwait                  3,127,000
Libya                      1,509,000
Nigeria                 2,417,000
Qatar                     1,966,000
Saudi Arabia       11,530,000
UAE                       3,380,000
Venezuela          2,725,000

This totals:            37,405,000  barrels per day from OPEC

Now global total oil production is 91,331,000 barrels per day

Thus:-

37,405,000 / 91,331,000 = 40%

OPEC accounts for just over 40% of Global Oil Production. This really is Big Oil.