OPEC agrees to sell their oil in dollars and this exchange with the USD as the oil currency keeps the faith in the value of the dollar.
Every country depends on oil. Which means every country has to have substantial dollar reserve so they can be able to purchase oil every year.
if oil costs less- every country would buy less dollars from the us.
The lesser the demand of the USD- the lesser its value.
Oil prices drop. So US can print more money which means they can buy all that oil and than demand more money for the oil.