Oil is quoted and traded in all kinds of currencies. It tends to be exchanged in dollars for the same reason that when Lufthansa bought Airbus jets in the last decade, they did the deal in dollars: Fedwire was, until very recently, the world's only international real-time high-volume electronic money transfer system. (Now Europe has a competitor.)
The Hussein euro thing is a conspiracy. In what currency do you think British North Sea oil being shipped to Europe is paid for?
The United States did not invade Iraq because it was threatening to switch to Euros. If anything, American banks would have been thrilled at the prospect of new FX revenues.
> Oil bourses trade in dollars
Oil bourses? Oil futures are mostly traded on exchanges, but oil the commodity is mostly traded over the counter. It's privately negotiated. Those private negotiations reference whatever they want to, whether Brent in USD, Brent in EUR, WTI in USD, COM in JPY, et cetera.
These exchanges trade what is referred to as 'light- sweet' crude oil and a single contract, or 'lot', calls for the purchase or sale of 1,000 barrels of oil. Traders can buy and sell oil for delivery several months or years ahead'