No you don't need to implement OCS in 100 locations.
Normal Audio & Video Calls are Peer to Peer so clients connect directly to each other
This behavior changes when doing Conferences (so this must be evaluated)
You should however maybe install Mediation Servers and Gateways in remote offices depending on how many countries you are talking about to integrate with current PBX centrales (but that is a bit hard to say without knowing your situation)
Quality of Experience server will collect quality of Audio & Video calls and show what the quality actually is, then you can progress with appropriate actions (typically you do this in pilot phase and in production phase)