Brent crude LCOc1 futures fell $2.83 to settle at $80.26 a barrel, a 3.41 percent loss, after hitting a low of $79.80, its weakest since September 24.
Oil prices slumped to more than two-week lows on Thursday as global stock markets fell, with investor sentiment made more bearish by a bigger-than-expected build in US crude inventories.
OPEC cut its forecast of global demand growth for oil next year for a third straight month on Thursday, citing headwinds facing the broader economy from trade disputes and volatile emerging markets.
Again, U.S. West Texas Intermediate (WTI) crude was down by 41 cents, also about half a percent, to 74.55 dollars a barrel, after rising almost 1 percent in the previous session.
"The clear risk-off mode that we are seeing across all markets is also hitting oil and those previous supply concerns have simply evaporated", said Michael McCarthy, chief market strategist at CMC Markets in Sydney.
"We will take care of them", Trump told reporters when asked about the decision of some countries like India and China to continue to purchase oil from Iran. "The large increase in overall crude oil inventories and a decent-sized increase at the Cushing, Oklahoma, delivery hub is a negative for prices, as it eases some of the supply crunch worries that have crept into the market", said John Kilduff, a partner at Again Capital Management in NY.
US crude inventories USOILC=ECI rose 6 million barrels last week, the Energy Information Administration said, more than double analysts' expectations of a 2.6 million-barrels increase. Analysts had expected an increase of 2.6 million barrels.
Industry and government data on US crude inventories will be delayed by one day this week because of Monday's US Columbus Day holiday.
In the United States, almost 40 percent of daily crude oil production was lost from offshore US Gulf of Mexico wells on Tuesday because of platform evacuations and shut-ins ahead of Hurricane Michael. The cuts represent 718,877 barrels per day of oil production.
While production has been cut because of the hurricane, "down time is expected to be brief and Gulf of Mexico output now accounts for a comparatively small portion of total USA production", Jim Ritterbusch, president of Ritterbusch and Associates, said in a note to clients.