Thanks for your support. I'll keep an eye, monitoring at different intervals and see how it goes. Can I assume that I can run the vmstat command if the apache server is unresponsive (if OOM for example) and still receive stats, or will the server fail to respond?

There's no guarantee of that; the OOM killer can target any process, including your ssh login. It tries to target the processes causing the trouble, but that can be surprisingly difficult for a machine to decide.

Then again, if your machine is OOMing then you don't need vmstat to tell you that you have a problem. At that point, ps auwx is probably more useful to show what processes are consuming memory.

Great, thanks again for all the advice/assistance.

FWIW, clamd can be a memory hog. When I ran it on a smaller linode, i had to disable quite a few of the databases it uses to make it fit in memory.

for me, it's currently running using ~300Mb of memory...

clamav 1932 0.1 14.6 362836 301104 ? Ssl Aug01 109:19 clamd