The o-rings that ship on ROTOR ALDHU 24 MM cranks are to help account for the different variances of production. Those variances include, but are not limited to, the width of the bottom bracket shell in the frame, the bottom bracket bearings and the crankset (arms and axle).
The o-rings go against the bottom bracket bearing cover. This will place them in between any spacers you use and the bottom bracket bearing cover.
However, please note the o-rings aren’t always necessary. We suggest using them initially, but if you find the crank is binding, please feel free to remove them.