Yeah, Committed_AS is the field to look for. What Robert said, it is a 99.99% guarantee that the system will not OOM if all the memory requests are granted and allocated by that kernel at that particular instant.
From kernel source.
Committed_AS: The amount of memory presently allocated on the system.
714 The committed memory is a sum of all of the memory which
715 has been allocated by processes, even if it has not been
716 "used" by them as of yet. A process which malloc()'s 1G
717 of memory, but only touches 300M of it will only show up
718 as using 300M of memory even if it has the address space
719 allocated for the entire 1G. This 1G is memory which has
720 been "committed" to by the VM and can be used at any time
721 by the allocating application. With strict overcommit
722 enabled on the system (mode 2 in 'vm.overcommit_memory'),
723 allocations which would exceed the CommitLimit (detailed
724 above) will not be permitted. This is useful if one needs
725 to guarantee that processes will not fail due to lack of
726 memory once that memory has been successfully allocated.
It is declared as a struct in source and is used in the function _vm_enough_memory() to see whether a process can grow in memory or not.
To cut short, it is a pretty good parameter to watch for memory issues.