From the BP website....
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Also interesting what they add about ethanol, since it contains the OH group (being an alcohol) it provides oxygen through this OH group....

"......This is BP Unleaded 91 with up to 10% ethanol. Because ethanol has a higher octane the addition of ethanol increases the octane to 94 typically so potentially there are economy and power benefits. However because the ethanol contains oxygen there can be up to 3% reduction in fuel economy to offset the power....."

I'm not sure the OH group really adds to the thermodynamics - since it will ultimately sop up H atoms to form H2O in the end (exhaust).
What I'm trying to say is....the fact that it "adds oxygen" is possibly a misnomer, since it adds OH, but this doesnt participate in breaking C-H's and making C-O&C-O2 where your main hp (enthalpy) gains are made.

I run Ultimate or Shell's equivalent V-Power in my cars - interesting read though.