New York's main contract, light sweet crude for delivery in December, shed 29 cents to $85.28 a barrel and Brent North Sea crude for December delivery fell 37 cents to $108.70.
"The oil market is falling on fears of slowing growth and the possibility of going off the fiscal cliff," said Phil Flynn, energy analyst at 321energy.
Rival US politicians must reach a deal to avoid a fiscal cliff of deep spending cuts and huge tax hikes which would come into force on January 1 and which observers say would tip the country back into recession.
Eurogroup head Jean-Claude Juncker said Monday that Greece had made progress on its debt bailout targets but eurozone finance ministers will have to meet again on November 20 to clear the way for its next aid tranche.