In the nitrous world, NOS is probably the most famous manufacturer of Nitrous for street racing, so the brand name has become synonymous with the product even if it is made by another brand!
So... calling nitrous "NOS" is the same as calling a hoover a hoover. However, I can't find a reason anywhere to call it NOX
N2O on the other hand is also correct; indeed in Fast and Furious and other films it is called N2O
I was looking at Wikipedia and it said that NOS came from Nitrous Oxide Systems http://en.wikipedia.org/wiki/Nitrous_Oxide_Systems
and the initials caught on as a generic name for nitrous oxide (N2O)