Rep. of the Congo joined it in 2018 (4)

I believe the answer is:

opec

(Other definitions for opec that I've seen before include "[CRUDE OIL] suppliers?" , "Organisation intended to control the production and sale of petroleum" , "Cartel of oil-producing countries" , "price-fixers?" , "Group of petroleum producing countries, formed in 1961" .)

I've seen this clue in The Wall Street Journal.

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