Possible Clues:

• Group with oil barrels
• Grp. of oil producers
• Grp. with many Mideast members
• Fuel cartel acronym
• Grp. with barrels
• Ecua. rejoined it in 2007
• Oil gp.
• Grp. affecting gas prices
• Cartel since 1960, briefly
• Org. whose product is measured in barrels