The "OOM Killer" or "Out of Memory Killer" is a process that the Linux kernel employs when the system is critically low on memory.
Whenever out of memory failure occurs, the out_of_memory() function will be called. Within it the select_bad_process() function is used which gets a score from the badness() function. The most ‘bad’ process is the one that will be sacrificed.
The above listed criteria mean that when selecting a process to kill the OOM Killer will choose a process using lots of memory and has lots of child processes and which are not system processes. An application such as Apache, MySQL, Nginx, Clamd (ClamAV), or a mail server will make an good candidate.