Reserve group, in brief? (4)

I believe the answer is:

opec

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

I've seen this clue in The New York Times.

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