Optic neuritis is just one thing that can affect the vision of someone with MS. There are other vision problems that sometimes people will call "optic neuritis" when they really aren't. Optic neuritis is a swelling of the optic nerve. It can cause pain (but not always), loss of color vision, a paling or distortion of color in that eye, sometimes loss of peripheral (side) vision, among some other things.
Other vision problems might be blurred vision, or nystagmus -- where the eye seems to "jump around", or difficulty focusing. This isn't ON, but can happen with MS.
I'm a wimp about my eyes. If there is anything wrong, I'm off to an opthamologist (a doctor whose specialty is vision and disease/injury that can affect vision, not an optometrist who can fit folks for glasses).
...I am not a doctor, nor health professional, and don't pretend to be one, here.....