Short answers, pulled from the story.
The Organization of Petroleum Exporting Countries began its life in Baghdad on the 14th of September 1960. Five nations gathered to sign a pact that would change global energy history: Iran, Iraq, Kuwait, Saudi Arabia and Venezuela.
OPEC was founded as a counterweight to previous Seven Sisters cartel of multinational oil companies because these dominant Anglo-American firms controlled all operations within exporting countries before OPEC existed. Oil-exporting countries eventually formed OPEC as a response to unilateral price cuts by multinational oil companies and to secure better prices for their crude oil.
The organization chose Geneva, Switzerland initially but moved to Vienna, Austria on the 1st of September 1965 after Switzerland declined to extend diplomatic privileges. Austria offered attractive terms to attract international organizations like OPEC while Switzerland attempted to reduce its foreign population.
In October 1973, Organisation of Arab Petroleum Exporting Countries declared significant production cuts and an oil embargo against United States and other industrialized nations supporting Israel in Yom Kippur War. This resulted in a sharp rise in oil prices from US$3/bbl to US$12/bbl and triggered global economic recession with unemployment and inflation surging simultaneously.
As of January 2024, OPEC has 12 member countries consisting of five Middle East West Asia, six Africa one South America. According U.S. Energy Information Administration EIA OPEC combined rate oil production including gas condensate represented 44% world total 2016.