Background: Diagnosis of multiple sclerosis (MS) may sometimes be ascertained at the time of optic neuritis (ON) but other times require the advent of new disease activity. Objectives: The aim of this study was to examine the predictive value of optical coherence tomography (OCT) and visual evoked potential (VEP) measurements of the non-symptomatic, fellow eye of ON patients, for conversion to MS. Methods: This is a prospective cohort study in patients with acute ON. OCT thickness measurements of peripapillary retinal nerve fiber layer (pRNFL) and macular ganglion cell layer–inner plexiform layer (GCLIPL), and multifocal (mf) VEP and full-field (ff) VEP, were performed. Univariate and multivariate Cox regression examined the value of predictors for the conversion to MS. Results: A total of 79 unilateral, acute ON patients, with no MS diagnosis or prior demyelination, were included. Of which, 28 patients developed MS during follow-up. Inferonasal GCLIPL, mean GCLIPL, and pRNFL thickness significantly predicted MS development in multivariate analysis (hazard ratio (HR) = 0.922–0.939, p = 0.0172–0.021). MfVEP mean latency (HR = 1.052, p = 0.006) only predicted MS conversion in univariate analysis. No significant predictive value was shown for the other parameters (p > 0.2). Conclusion: While both mfVEP and OCT are useful tools in the evaluation of acute ON patients, only OCT measurements of fellow eyes may serve as an independent predictor of MS development.