Why?
Because nations could now buy oil with Euros and this would free them from having to have only dollar cash reserves for purchasing oil. Currently all oil is purchased with dollars which keeps the dollar in high demand around the world.
Theoretically, with oil-for-Euros the dollar demand drops.
That's what is not clear at all. Are there so many dollars held beyond the US because international markets use dollar price quotes, or do markets use dollar quotes because there are so many dollars in so many hands? Few "nations" buy oil; most is bought by private interests. I can take my euros right now and buy oil on international markets (quoted in dollars), and if need be use an exchange rate market. A Chilean buys oil, priced in dollars, from Mexico, and passes a Mexican exporter a check written in pesos drawn on a bank based in Monterrey. And has the use of dollar units for oil hurt the value of the euro, or has the hardness of the euro made it a candidate for serving as a unit of trade? Foreign exchange reserves are most often in dollars due to its hardness, not the price quote unit. How much oil does Brazil import? How much does Ecuador? Saudi Arabia? Why are their reserves significantly in dollars? (By the way, did the shift from price quotes in ounces of gold to quotes in British pounts or US dollars hurt the value of gold? Did the shift of international markets from British pounds to dollars hurt the value of the pound?)