Oil fell more than $2 to below $52 a barrel today after OPEC decided to wait until mid-December to make another cut in output to try to defend sagging prices.
U.S. light crude for January delivery was down $2.53 at $51.90 a barrel by 0937 GMT.
Oil had settled at $54.43 on Friday after a shortened post-Thanksgiving holiday session. On Nov. 21, it touched a three and half year low of $48.25.
London Brent crude was $2.62 lower at $50.87 a barrel.
"The markets are discounting OPEC's decision to stand put by selling off," said Edward Meir, analyst at broker MF Global.
"When it comes to calibrating supply and demand to fit the new post-September economic realities, OPEC seems to be in a state of denial," he said in a research note.
Oil is down by almost two-thirds from a peak of more than $147 a barrel in July. Prices fell almost 20 percent in November and 32 percent in October, their biggest monthly fall ever, despite OPEC's around 2 million barrels per day cutbacks.
"FAIR PRICE"
A global economic slowdown that has tipped a growing number of countries into recession has caused sharp falls in demand for oil.
But OPEC's Gulf producers want to see strict compliance with the producer group's existing output curbs of 2 million barrels per day (bpd) before agreeing to any more.
The Organization of the Petroleum Exporting Countries (OPEC) meets next in Algeria on Dec. 17.
Saudi Arabia on Saturday pointed to $75 a barrel as a "fair price" for oil, the first time in years that the world's biggest exporter has identified a target for crude prices.
Saudi Oil Minister Ali al-Naimi in Cairo cited this price as necessary to keep more expensive new projects at the margins of world supply on track.
"I believe $75 is the price for the marginal producer," he told reporters in Cairo.
On Sunday he told the Saudi-owned al-Hayat newspaper the effect of OPEC's existing cuts was still not clear.
In Cairo, OPEC ministers discussed how much more they needed to cut. Most, including Gulf producers led by Saudi Arabia, saw the need to trim another 1 to 1.5 million bpd.
OPEC Secretary General al-Badri told reporters in Tehran on Monday: "It will be a good amount, a good quantity," without naming a specific figure.
Analysts said OPEC's delay could prove costly.
"The longer OPEC waits to cut supplies, the higher stocks rise and the longer we think it'll take for fundamentals to tighten once the tide does turn," Jan Stuart, an economist with UBS in New York, said in a weekly U.S. oil data report.