from descriptions on Amain...

The 24KO features a bigger blank with more material in the stator. This makes it harder to heat soak the stator, and able to handle more timing and higher gearing. These qualities make the 24KO a better choice for high load applications such as any boosted class, heavier vehicles and short tracks.

The 24K features a smaller blank with less material in the stator that produces more power than the 24KO, but is easier to heatsoak. This motor is faster than the KO initially, but depending on gearing, timing and load it could fall off more at the end of the run when compared to the KO. Because of this, the standard 24K is recommended for blinky applications.