I would say if OAS gets disabled then this fact has preliminaries for example in the local windows Application log. I suggest you look into that log on those clients.
Where is that log at?
One way to get to it is right click My Computer, select Manage, in the appearing window left pane double click Event log and then select Application underneath. Log content apears to the right. Find McLogEvent entries and look into them if one of them contains an error message.