Oil is bought and sold in US dollars across the globe. When the dollar gets stronger (as it has over recent months), it makes oil more expensive to buy in countries outside the US. That, in turn, weakens worldwide demand and further puts downward pressure on oil prices.
“While the barrel of oil is cheaper here in the US – for the US consumer – it's going to be more expensive for people overseas,” Phil Flynn, senior market analyst at the Chicago-based PRICE Futures Group, tells NPR. “And that's really adding to some of the economic woes that they're having right now.”
Even though global oil prices are falling, they’re falling less for countries with currencies that are weaker than the US dollar.