It's kinda funny story, at first it was founded by Iran, Iraq, Kuwait, Saudi Arabia and Venezuela to oppose anglo oil giants and their states doing things like the Iran coup. Problem is, despite it being a real deal shameless cartel, it's too weak to truly monopolise oil production and too weak and disorganised to have any kind of common defense against US military (we can observe effects on the case of Iraq, Syria, Libya). and thus later it was forced to enter an uneasy pact with USA, resulting in creation of the petrodollar standard - which is one of the main reasons for US hegemony. That said, they have around half of the world oil production, which is enough to have giant economical influence.

So if chauvinist like Sanders is frothing at his mouth at them, it means either that they just decided that without consultation with US or ignored the US warning to not do it. Because lowering production in brewing of westernwide energy crisis will not only cause much trouble for US and EU but also strenghten other exporters though rising prices. Chiefly Russia.