If the dollar fell x% since Friday then the probability that OPEC will switch to euros can be calculated from:

current value of $ = $'s value (say) at noon on Friday * Prob{OPEC will not abandon $} + $'s value if OPEC abandons it * (1- Prob{OPEC will not abandon $}),

provided "$'s value if OPEC abandons it" can be assigned a quantity. E.g. if "$'s value if OPEC abandons it" = 0 (for demonstrative purposes) then:

Prob{OPEC will not abandon $} = (current value of $) / ($'s value at noon on Friday) = 1 - x.

One of the perceived reasons for the Iraqi war starting is because Saddam wanted to switch from Dollars to Euros. We couldn't let that happen, now could we?