According to articles like this there seems to be a move towards Europe saving billion dollar deals with Iran by switching to Euro for crude purchases. Apparently to avoid dollar sanctions on Iran having an impact on these deals, and to help save the Iranian nuclear deal.
In my opinion the importance of OPEC's USD trade exclusivity is grossly underestimated by most. Without the OPEC oil prop, and Sunni dominance of this arrangement, the entire global order is threatened.
Central banks must maintain dollar reserves to purchase energy and oil for manufacturing. Without that incentive, dollar demand drops. This would, for a while, be absorbed by the effective 'Quantitive Tightening' currently being executed at the Fed. This could result in artificially diminished effects of dollar liquidity drainage from the global system. A 'kick the can down the road' effect might happen. On the other hand, the Fed's insistence on draining dollars out of global circulation would be facilitated by any European endeavours to enhance the status of Euro as a reserve petro-currency.
Historically, any attempts to switch off the USD have been met with overt and covert hostility (Venezuela, Russia, Iran, Iraq, Libya). However, US domestic policy (Fed tightening) and foreign trade policy (reduce Asian exporter trade balances) seem to be in line, at least temporarily, with what Europe allegedly aims to do. The current US administration seems to be in favour of reversing globalization, bringing manufacturing back home, avoiding overseas military expansion, and focusing on domestic development. Is this now a strategy/direction being supported by Europe? Or is this a challenge that will result in some dramatic modifications to US foreign policy towards Europe and the world?