Saudi Arabia is also part of BRICS and it also a big source of oil, maybe not bigger than Iran but it do have big resources, and talking about Iran going against the US dollar is not something new, because back in time, NIOC has made many tries to avoid using US dollars but you know they were forced. Well, the same thing would have happened with Iran which happened with Iraq, but now they have taken the support of BRICS.

Talking about Russia, even if they are a big source of oil and are good competitor of the USA, but still they might have to trade using US dollars while making trades with the US. Overall, I don't think US will be in any danger if these countries will starts to make deals in local currencies, that's because they ways to make deals in local currencies are full of difficulties and these difficulties will increase more by the ones who might be in danger due to this.