When you go the page for the Hypervisor in your OnApp control panel you see the RAM showing up as zero, even though you know it isn't zero.
How to Diagnose
to make sure the error is that your control panel is actually reporting the wrong data you need to check the xml page from the API call. To do this you'll need to go to this page in your browser:
<Your Control panel URL>/settings/hypervisors.xml => then search for that specific HV on that page
You'll want to look for this entry on the page: <free_memory type="integer">183826</free_memory> If that value shows anything other than 0 than you know your control panel is reporting it wrong. If this value also shows 0 that means there is either a problem with snmpd or you are actually out of RAM. Please contact support in this situation.
The reason you are seeing this is because you most likely have memory guarantee enabled for this HV. When memory guarantee is enabled the memory show in the UI is calculated like this:
<actual free memory on the HV > - sum(memory)_all_stopped_vms_on_the_hv.
If this value is less or equal 0 - then you got 0 free memory.
So in order to change this value than you need to disable memory guarantee or else clear up some of the stopped VM's. To turn off memory guarantee you just need to go to the Hypervisor zones - and edit the proper zone. You will see the memory guarantee button there.