Many vulnerabilities are operating system specific. Information about the OS of all hosts in a network represents a valuable asset for network administrators. While OS detection in small networks is an easy task, expanding the same process on a large scale becomes a challenge. The weak performance, high speed traffic and large amount of hosts for OS detection are issues to overcome. In this paper we propose a flow based framework for large scale OS detection. Furthermore, we describe the framework implementation into a flow probe, provide performance comparison and share remarks on deployment in a real world network.