raytrace

There are at least 96 oil producing countries in the world ([1]). It makes sense to trade oil with USD across the board instead of fragmenting the oil market into 96 different sub-markets (oil to each country's local currency).

Now, the buyer and seller can of course convert USD from/to their desired currency on FX markets. Oil trades tend to be big, so that would cause demand for USD liquidity. That demand, and the fact that these dollars are effectively "locked" inside the FX markets (reducing supply), would make the dollar price go up (everything else being equal), hence strengthening the dollar.

[1] https://en.wikipedia.org/wiki/List_of_countries_by_oil_produ...