I believe, and I could be very wrong, that it kills the largest memory process. The problem is that it may not be the problematic process. You could have a well behaved DB that is using a stable 80%, and a runaway process that rapidly fills the remaining 20%, and OOM will kill the DB, and let the out of control process keep consuming memory until it becomes the largest. Then your DB is down and you don't known why.

I like the idea of just killing processes at random though. Determinism is overrated.