Economists often cite OPEC as
a textbook example of a cartel that cooperates to reduce market competition, but one whose consultations are protected by the doctrine of
state immunity under international law.
[...]
At various times, OPEC members have displayed apparent
anti-competitive cartel behavior through the organization's agreements about oil production and price levels.
[155] In fact, economists often cite OPEC as a textbook example of a cartel that cooperates to reduce market competition, as in this definition from
OECD's
Glossary of Industrial Organisation Economics and Competition Law