OOM, This toilet cleaner smells good

As commented above, the OOM killer is not designed to fix the swap-trashing problem you describe, the OOM killer is designed to fix the problem when your system is so active that you run out of *both* RAM and swap. Only then will OOM killer be activated at all.

For your problem, the solution is to greatly reduce you swap size, reduce the 'swappiness' value of your kernel and also implement the "don't cache disc access by this process" feature and mark you overnight cron jobs with that bit.