By Nicole Friedman 

U.S. oil prices posted their biggest one-day gain in two years on Monday as traders locked in profits on bearish bets following last week's selloff.

But many market watchers were skeptical that Monday's gains signaled that oil prices had reached their bottom, pointing to global supplies that continue to overwhelm demand.

Oil prices tumbled to multiyear lows after the Organization of the Petroleum Exporting Countries decided on Thursday to maintain its production quotas, rather than agreeing to support prices by lowering its output target.

The group's decision sent shock waves through financial markets that continued to be felt on Monday. Shares of some U.S. oil producers added to last week's declines, highlighting concerns about these companies' ability to operate expensive shale-oil fields amid plunging prices. Bonds of low-rated energy companies also sank, reflecting fears that some producers will default if prices fail to recover.

Many investors and analysts believe with OPEC on the sidelines it will take cutbacks by companies in the U.S. and Canada to bring supply and demand in line and pull the market out of its swoon. That day may not come until deep into 2015 or beyond, some analysts say.

"The era of $100 [a barrel] oil is over," Citigroup said in a note. "Oil prices appear to be falling rapidly to--if they haven't already reached--production costs."

On Monday, light, sweet oil for January delivery shot up 4.3% to $69.00 a barrel from Friday's close, recovering from a more-than five-year low below $64 a barrel hit in overnight trading. It was the biggest one-day percentage gain for the U.S. benchmark since August 2012. Brent futures, the international benchmark, gained 3.4% to end at $72.54 a barrel.

Investors said much of Monday's activity was driven by traders closing out profitable bets on falling prices or opening positions that would protect them should the market rebound. Both the U.S. benchmark and Brent slumped Thursday and Friday to fresh multiyear lows, posting two-day losses of about 10%.

"Today's a little bit more of a rebound from Friday's plastering," rather than a long-lasting change in direction, said Kyle Cooper, analyst at IAF Advisors in Houston. "We got [to five-year lows] really quick, and it's not an entirely bad thing to take some profits."

Roland Austrup, who manages $54 million as chief executive of Integrated Managed Futures Corp., said his funds posted 6% gains in November largely due to a bet that oil prices would fall. After last week's selloff, he kept his wagers on lower oil prices but used options to reduce losses in case prices rose.

"Anytime you get a big significant move like this, it's prudent to hedge, " Mr. Austrup said. "It certainly doesn't reflect a view that it's over...There's no impetus for prices to go up."

Volatility in the oil market has caught out some funds. Brevan Howard Asset Management LLP plans to close its $630 million commodity hedge fund after the fund suffered heavy losses in energy markets in September, people familiar with the fund said Saturday.

The slump in oil prices is pressuring energy companies, and analysts and bankers expect a string of deal-making as highly levered companies become attractive for acquisition.

Halliburton Co. is in the process of buying rival oil-service company Baker Hughes Inc. Weatherford International PLC on Monday said it agreed to sell its engineered-chemistry and drilling-fluids businesses to an affiliate of Berkshire Hathaway Inc.'s Lubrizol unit. Weatherford said it expects to use proceeds from the deal, which is expected to close before the end of the year, to pay down debt.

Some investors are looking for signs that slumping oil prices could spark higher demand, and U.S. employment data on Friday will be a key indicator, said Bob Yawger, director of the futures division at Mizuho Securities USA Inc. Higher employment can spur more gasoline demand, as more commuters drive to work. The national average price of retail gasoline was $2.77 a gallon Monday, down 50 cents from a year ago.

Upcoming macroeconomic reports "will be looked at as a demand indicator and the energy market will adjust itself accordingly," Mr. Yawger said.

But without action from OPEC, the market will remain oversupplied, said Barclays in a note. Barclays lowered its oil-price forecasts for this year and next after the OPEC meeting, as did BNP Paribas SA and Citigroup Inc. Barclays now expects Brent prices to average $72 a barrel next year, while BNP forecasts $77 a barrel and Citi has called for $80 a barrel.

"It will have to endure a volatile adjustment period while non-OPEC supply, demand and even some OPEC producers adjust," analysts at Barclays said.

January reformulated gasoline blendstock, or RBOB, rose 5.34 cents, or 2.9%, to $1.8810 a gallon.

January diesel gained 5.12 cents, or 2.4%, to $2.2124 a gallon.

Matt Wirz, Laurence Fletcher, Chiara Albanese and Michael Calia contributed to this article.

Write to Nicole Friedman at nicole.friedman@wsj.com

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