By Henning Gloystein
Brent crude futures, the international benchmark for oil prices, were at $75.36 per barrel by 0708 GMT, down 52 cents, or 0.7 percent, from their last close.
U.S. West Texas Intermediate (WTI) crude futures were at $65.87 a barrel, down 49 cents, or 0.7 percent, from their last settlement.
The Organization of the Petroleum Exporting Countries (OPEC), together with some non-OPEC producers, including Russia, started withholding output in 2017 to reduce a global supply overhang and push up prices.
OPEC on Tuesday said the oil market outlook in the second-half of 2018 was highly uncertain, and warned of downside risks to demand.
The group is due to meet on June 22 in Vienna, Austria, to discuss future production policy.
Dutch bank ING, however, said that some OPEC members would "struggle to push production back to October 2016 levels".
The rising stocks are in part a result of the surge in U.S. crude oil production, which has jumped by 28 percent in the last two years, to a record 10.8 million barrels per day (bpd).
With output in Russia rising back above 11 million bpd in June and Saudi production climbing back above 10 million bpd, supplies from the top three producers are increasing.
"With rising production from U.S. shale adding to oil's woes and reviving oversupply concerns, further downside could be a possibility in the short to medium term," Otunuga said.