OPEC, in its December Monthly Oil Market Report, estimated world economies will grow by 4.8 percent next year, down from 5.2 percent in 2007, and said there were "considerable downside risks" to the outlook.
"Prospects for 2008 are increasingly clouded by the expected slowdown in the U.S. economy and other OECD regions, and by continued turbulence in financial markets in the wake of the deepening subprime mortgage crisis," the report said.
"The improving geopolitical situation and slowing economic outlook should help to further ease the pressure on the market."
OPEC also forecast world oil demand would grow by 1.3 million barrels per day next year, steady from the previous estimate and much lower than some projections.
The group, which pumps more than a third of the world's oil, is more pessimistic about demand than the International Energy Agency, which earlier on Friday raised its forecast for demand growth next year to 2.1 million bpd.