The uncertainty is putting a damper on things like tech investment and lower trade tariffs, which usually help economies grow.
In the US growth is expected to dip from 2% in 2026 to 1.7% in 2027, with inflation hitting around 4.2%.
Europe's outlook is even tougher: GDP growth there could drop to just 0.8% in 2026 thanks to high energy costs, though a small rebound might follow.
China's economy seems steadier at 4.4%.
The OECD suggests governments offer targeted, time-limited support and calls on central banks to remain vigilant.