It isn't.

On Linux, the OOM generally weighs 2 things, and based on those, starts killing processes until the system dies or goes back to life.

The 2 things it weighs are memory footprint and how recent the process is.

The reasoning is that, by killing a recently spawned process that is hogging a lot of memory, you will probably kill an unruly app that a user started and not a critical system component. Even in the case of servers with databases that use hundreds of GB of memory, it should in theory not kill them if there's a recent process that has quickly hogged a lot of ram.

Of course, it can still be a bit of a hit or miss (after all, it's a last resource before crashing, and you can't afford much complexity). But for the most part, it works well enough.

Edit: I actually looked at the code. It doesn't check if the process is recent. I don't know where I read that it did. Maybe it was the OOM code for something else.

There are still ways to mark an app as non oom killable, like a critical DB process. But it needs to be manually done.