New York's main contract, light sweet crude for December delivery, was up 51 cents to $81.94 a barrel.
Brent North Sea crude for December delivery advanced 15 cents to $83.30.
A declining greenback, whose value earlier Monday against the Japanese yen edged closer to its all-time low below the 80 yen line hit in April 1995, was proving irresistible to oil traders, analysts said.
"The dollar has weakened some, and oil is following in the opposite direction," said Victor Shum, senior principal of energy consultants Purvin and Gertz in Singapore.
The dollar traded at 80.76 yen at 0150 GMT after plunging to a new 15-year low of 80.21 yen in Asia earlier Monday.
The greenback also slipped against the euro. One euro bought $1.3966 in morning Asian trade, up from $1.3929 in late US trade Friday.
Traders were apprehensive ahead of the US Federal Reserve Federal Open Market Committee meeting starting Tuesday, said Shum.
"The market expectation is of quantitative easing number two, and that is causing a weakening dollar," he said.
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