Definition from Wiktionary, the free dictionary
Network Operating System
Usage notes 
- (Not Otherwise Specified): used as the last qualifier in the definition of a class or category, as a punctuation period that restricts further narrowing of that specification, such as in the UN number for the transportation of dangerous goods; e.g., UN1956 is defined as "Compressed Gas, N.O.S."; whether it is flammable or inert is not specified, so it could be either.