The use of a single OCS server for multiple clients is quite possible, as long as this server is accessible from everywhere (in fact, it is even designed for that !).
So, if the OCS server is today hosted at your client site, it would be better to export it (in data-center for example, to avoid dependence on the client's security rules [firewall, etc.])) and to use different profiles / users / groups of machines.
I don't use GLPI but I think the link with OCS can in your case (multiple clients in OCS to a single GLPI) can be a difficulty to be considered.