For the first half of 2019, based on its outlook for non-OPEC production and global demand, and assuming flat OPEC production, the IEA said the implied stock build is 2 million bpd.
Output around the world has swelled since the middle of the year, while an escalating trade dispute between the United States and China threatens global economic growth.
On Wednesday, three sources familiar with the matter told Reuters that OPEC and its partners are discussing a proposal to cut oil output by up to 1.4 million bpd for 2019 to avert an oversupply that would weaken prices.
Since early October, the oil price has fallen by a quarter to below $70 a barrel, its lowest in eight months, which may protect demand to an extent, the IEA said.
"While slower economic growth in some countries reduces the outlook for oil demand, a significant downward revision to our price assumption is supportive," it added.
The agency raised its forecast for oil output growth from countries outside the Organization of the Petroleum Exporting Countries to 2.4 million bpd this year and 1.9 million bpd next year, versus its previous estimate of 2.2 million bpd and 1.8 million bpd, respectively.
The United States will lead output growth. The IEA estimates total U.S. oil supply will rise by 2.1 million bpd this year and another 1.3 million bpd in 2019, from a current record of more than 11 million bpd.