pure O2 will not work and i highly advise against trying it.
Using it will result in teh uncontrollable burn, note explosion, in teh combustion chamber, making possibly one of the largest anti personell grenades.
Atmo air doesnt explode, but burns in a controlled manner as flame propogation through the mixture of air and fuel is only 14% or so O2. SO there are alot of otehr elements getting in the way of pre flame formation.
In N20 applications, the dissociation of N20 into O2 and N on teh compression stroke absorbs alot of heat energy and damps the combustion process.
Just like higher octane fuels dont burn slower, they just go through pre-flame changes slower.