I believe the kernel OOM killer only fires when you're completely out of memory. But it's not unusual for swap thrashing to make the system unusable before that, forcing the user to either try to kill a likely culprit by hand, or hold down the power button.

systemd-oomd (and earlyoom, which Fedora used for a couple of releases before switching) try to detect that you're running out of RAM before the system becomes unusable, and kill a big memory user. It periodically killed Firefox before I bought some more RAM, which was a pain, but less of a pain than hard-rebooting the laptop.

The plans for Fedora to do this are public pages:

https://fedoraproject.org/wiki/Changes/EnableEarlyoom https://fedoraproject.org/wiki/Changes/EnableSystemdOomd