OPEC, and since 2016 the wider OPEC+ grouping that includes Russia and others, is a set of producers that coordinate output targets. Its members hold a large share of world production and the great majority of low-cost reserves, which is what makes coordination worth attempting at all.
The mechanism is a quota: an agreed production target per member, revised at ministerial meetings. Announcements move the market immediately, because they are statements about future supply from producers who can actually deliver on them. What follows the announcement is the harder question, because a quota is a promise and production is a measurement.
Compliance is the gap between the two, and it is genuinely difficult to compute for a reason worth understanding. Actual production is estimated by several independent sources, which disagree, and the group itself uses a set of designated secondary sources rather than member self-reporting, precisely because self-reported figures had an obvious incentive problem. Baselines are renegotiated, exemptions are granted, and voluntary cuts sit on top of group cuts. The result is that a compliance percentage depends heavily on which baseline and which production estimate you use.
The economics of the group are worth stating plainly because they are often described in moral terms. A producer cutting output gives up volume to raise price, and that is only rational if the price gain outweighs the volume loss, and if others do not simply fill the gap. Both conditions are contested at every meeting, which is why cohesion varies and why the group's history includes both effective coordination and open price wars.
The relevant analytical quantity is usually not the quota but spare capacity: how much a producer could bring back quickly if it chose to. A group producing at capacity has no policy lever left regardless of what it announces.