US benchmark ended Friday's session down $4.21, or 7.7 percent, at $50.42.
West Texas Intermediate crude for January settled down 7 cents to $51.56 a barrel on the New York Mercantile Exchange.
"Expected OPEC+ supply growth reversal starting in December and declines in Iranian shipments later in the year should also serve as an important development contributing to the widening of the Brent-WTI differential in 2019".
Norbert Ruecker, head of commodity research at Swiss bank Julius Baer, said oil had buckled after "a surprisingly swift and pronounced change in the market mood from shortage fears to glut concerns" while the world economy was also slowing down.
"After Friday's panic selling, the move to the downside may be somewhat overdone", Justin McQueen, DailyFX analyst, separately told UPI.
The Organization of Petroleum Exporting Countries and its partners will meet in Vienna.
"Considering the cost of carry associated with the recent re-emergence of a contango curve structure, we think taking flat price-risk may not be the best way to express our view. The market is now looking for a potential repeat", Hansen said. Overall, US oil prices are down 10 percent.
The argument against the production cut surrounds OPEC's market share.
Saudi Arabia raised oil production to a record high in November, an industry source said on Monday, pumping 11.1-million to 11.3-million barrels a day during the month.
Saudi Energy Minister Khalid al Falih on Thursday said the kingdom's output this month would of 10.6 million barrels per day. This Saudi announcement helped prices ease from peaks in early October.
The United States is not a member of OPEC and is not participating in the output reduction.
Saudi Arabia would like higher prices to fuel its economy, which contracted in 2017 and is projected to grow by just a little over 2% this year, according to the International Monetary Fund.
"There are many moving parts when it comes to political considerations, given the pressure from the USA president and the murder of Khashoggi", said Bjornar Tonhaugen, head of oil markets at Rystad Energy.
Oil markets opened cautiously on Tuesday, with many traders reluctant to take on large new positions ahead of the G20 gathering in Argentina this weekend and the OPEC meeting in Austria next week. "This would require a clear statement from OPEC that it is willing to cut production considerably", Commerzbank AG said in a note.
Friday's declines further ramp up the pressure on ahead of a much-anticipated meeting between the influential oil cartel and its allies in Vienna on December 6, when they are expected to announce that output will be curtailed.