The Paris-based intergovernmental group said on Tuesday that it now anticipates the global economy to grow by 3.9 per cent both this year and next, from a previous forecast of 3.6 per cent for both years.
As downside risks, the report cited persisting gestures of trade protectionism, the normalization of monetary policies in key countries, demographic changes and a decrease in mid- and long-term investment.
The OECD forecast the US economy would grow 2.9 percent this year and 2.8 percent in 2019, with tax cuts adding 0.5-0.75 percentage points to the outlook in both years.
In its interim economic outlook report, the OECD projects 3.9 percent global growth for 2018 and 2019, up from the November forecasts of 3.7 percent and 3.6 percent, respectively.
The latest report lifted growth forecasts for the US and eurozone economies.
Financial markets have been hit by fears of a trade war following U.S. President Donald Trump's move to slap tariffs on steel and aluminum imports.
The UK will be the slowest-growing economy in the G20 barring South Africa if the OECD's predictions come through, with growth well behind the 2.2 per cent average expected in the Eurozone or the 2.5 per cent annual expansion in the US.
Alvaro Pereira, the OECD's acting chief economist, said: "US steel and aluminium tariffs will raise costs and harm consumers, while not solving the global overcapacity problem". That was higher from a November forecast of 1.2 percent due to the broader global improvement.
For South Africa, the OECD has revised the expected GDP growth rate upward to 1.9% in 2018, and 2.1% in 2019 - higher than the growth rate now targeted by National Treasury.
The OECD said high inflation would eat into United Kingdom household income while business investment would slow in the face of uncertainty over Britain's future relationship with the EU.
By contrast, growth in the global economy is expected to strengthen further from its six-year high of 3.7% in 2017 to reach 3.9% in 2018 and 2019.
In Germany, growth is seen coming it at 2.4 per cent this year and 2.2 per cent next, up from 2.3 per cent and 1.9 per cent previously.
"Safeguarding the rules-based global trading system will help to support growth and jobs", it said.