As usual, google produces answers in less than 30 seconds.
In most states, it is legal to carry NO in your car. Texas has zero requirements about it, but most others require a permit to "legally" transport and/or a placard in the window stating there's pressurized gas inside to be 100% up-and-up. Some states the permit is free, others you have to pay. In ANY case, it is NOT legal to use it. Again, more variants about can't be hooked up, can be connected but not open, can't be in reach of driver, ect.
In California, like you'd expect, it's a no-go. You need a hazmat endorsed license to legally transport it.
To me, it's one of those things that's only illegal if you get caught. It's a lot harder to hide a bottle on a bike, and any cop who's been on the job for more than a week knows what a NO bottle looks like. Plus, how is a LEO going to know if you're using it or not? I've had other riders ask me if I had NO just from how fast the Vmax is stock.
I wouldn't worry too much about it.