The term 'Big Oil Supplier' can be summarized by the crossword solution of IRAN, which is a four-letter word. IRAN is a major oil supplier in the Middle East and is one of the world's top producers of crude oil. It has the second-largest proven reserves of any country in the world and is a member of the Organization of the Petroleum Exporting Countries (OPEC). IRAN is also a major exporter of natural gas and is a key player in the global energy market.