If someone has optical neuritis, will they develop ms?

Varies, but: Statistically, the literature varies from 25% to 65-70% incidence. Best data from England: optic neuritis plus one brain MRI lesion becomes clinically definite MS in 85% of pts by 14 years. So, obtain serial MRI studies if in doubt.

Sometimes: Optic neuritis is a common first condition for those later found out to have MS, but the first attack may be isolated and unrelated. Most will not do MS investigation until there is a second attack.