OPEC has been pumping near record levels since last year in an attempt to drive higher-cost producers such as U.S. shale firms out of the market. U.S. output has fallen though not by much, while production from Russia — another big non-OPEC player — has risen.
OPEC supply is likely to increase by 1 million barrels per day next year, Morgan Stanley said in a research note.
"Almost the entirety of added supplies in 2016 will come from Iran, Iraq and Saudi," it said.
Iran has promised to ramp up supply once nuclear-related sanctions are lifted on its crude exports. Tehran is expected to raise crude and condensates exports by as much as 700,000 bpd by the end of 2016.
"The news on the supply side continues to be that there's a glut out there. IEA on Friday saying that the oil glut will last until at least late 2016," Harris said on "Fast Money: Halftime Report." "Then you have the Iran deputy oil minister throwing oil on the fire, so to speak, with his comment about absolutely no chance of delaying the output increase."
Harris said as long as the price trend continues to the downside, he'll continue to stay short oil.
— CNBC's Ritika Shah contributed to this article.