I can't speak to the other eye, but I'm going to guess it's a side effect of the steroids or stress. Bi-lateral ON is actually not common in MS and I think it would be unusual for another attack to happen WHILE you're on steroids.

As far as ON, it usually hits a curve and recovery happens in reverse, the way it came. I don't know that steroids will halt the progression of sx once it begins but it should help your vision recover faster and avert further damage because of the reduction in inflammation. The only way to really chart progress once ON begins is by follow-up examination. The visual field tests will usually show improvement long before you yourself will see it.