I came across the same story “that Iran will construct an ‘oil bourse’ with oil priced in terms of Euros instead of dollars” as Tyler Cowen, and I dismissed its claims that such a system would “swiftly destroy the financial system underpinning the American Empire” for the same reasons:
As a first-order approximation, it doesn’t much matter whether Iran prices its oil in terms of dollars, Euros, or some other currency. At the beginning of each day, investors (including the OPEC nations) are holding their preferred bundle of currency positions. You might need to hold or receive Euros for a moment to make a transaction, but moving from the dollar to the Euro, or vice versa, can be done easily.
Currency markets are some of the most liquid markets in the world.