What is OOM Killer

The OOM killer (Out-Of-Memory killer) is a Linux kernel engine that frees up the RAM in case of exhaustion by terminating existing processes. 
When the limit of RAM on the VPS is completely reached, the OS kernel triggers the OOM killer which selects the process according to a specific set of rules and forcibly terminates it. The released memory is transferred to the OS kernel control, and then to other processes that need RAM resources. 

If you notice that your VPS does not have enough memory, try the following actions: 

1. Disable anti-virus software on the server. Antiviruses must constantly keep in memory the virus base, so disabling them can free up a significant amount of RAM. For example, disabling clamd in the ISPManager panel can free up about 600 MB of RAM. Please note that this method reduces the security level of your service, so we strongly recommend disabling antivirus software only as a temporary measure while optimizing/configuring the server.
2. Perform additional server configuration or optimize website scripts/database queries. To perform these actions, we recommend contacting experts in the field of administration or development.
3. If your server already has the optimal configuration, but the memory consumption still reaches its maximum, you can purchase additional memory in your personal account, in the “Services” section – “Available Addons”.