Russia sits in OPEC+ without being a member of OPEC
Background
This standing fact sits behind the decision by seven producers to hold December output at the September level. OPEC stands for the Organization of the Petroleum Exporting Countries. Russia is not one of its members. Russia works with OPEC through the wider arrangement known as OPEC+, which brings in large producers from outside the organisation. Within that arrangement a group of seven countries - Saudi Arabia, Russia, Iraq, Kuwait, Kazakhstan, Algeria and Oman - handles the voluntary output adjustments and meets, usually online, to set its required production month by month. Of those seven, Saudi Arabia, Iraq, Kuwait and Algeria are OPEC members, while Russia, Kazakhstan and Oman take part only through OPEC+.
In plain words
OPEC Organization of the Petroleum Exporting Countries — A group of oil exporting states that coordinates production policy.