With KM, but not enough runes to obliterate:
Every second you wait, there is a chance that KM is overridden. So how long is it worth waiting before the override renders waiting not worth it?
KM has 6 ppm, and fully raid buffed most dks have around 50% haste. That's 9 actual ppm, or .15 pps. Therefore, every 6.666 seconds, on average, you will get one KM proc. If FS gets an x boost of damage from KM, obliterate gets about 2x. Therefore, with average luck, waiting the full 6.666 seconds makes you break even, whereas waiting less becomes a bigger gain the less time you wait, assuming you get KM to actually proc. This is also assuming you continue to not allow the other resources to overcap.
So that basically means that its worth waiting 6 seconds with a KM proc to use on OB. If you wait LONGER then 6.6 seconds it`s considerd a DPS loss to not use Frost strike on a KM proc.