Here I am just explaining Network OS which should make you to clarify the difference between NOS and OS :
Network Operating System is an OS that is oriented to computer networking. For example, one that runs on a server and enables the server to manage data, users, groups, security, applications, and other networking functions.The network operating system is designed to allow shared file and printer access among multiple computers in a network, typically a local area network (LAN), a private network or to other networks.
Network Operating Systems can be embedded in a router or hardware firewall that operates the functions in the network layer (layer 3) of the OSI model.
Examples of NOS:
1. JUNOS, used in routers and switches from Juniper Networks,
2. Cisco IOS (formerly "Cisco Internetwork Operating System").
3. TiMOS, used in routers from Alcatel-Lucent
4. VRP (Versatile Routing Platform), used in routers from Huawei
5. RouterOS, software which turns a PC or MikroTik hardware into a dedicated router
6. ZyNOS, used in network devices made by ZyXEL.
7. ExtremeXOS, used in network devices made by Extreme Networks. Also called EXOS.