We previously reported on a 16-week, double-blind, randomized placebo-controlled trial (RCT) using 3 grams per day of N-acetyl cysteine (NAC) (1.5 grams twice per day) in 44 participants (aged 18–70) with DSM-5-diagnosed obsessive-compulsive disorder (OCD). We now report on an analysis of age, severity and duration of illness, OCD presentation type, baseline anxiety and depression scores, as well as the use of antidepressant medications as potentially modifying factors. Results revealed a significant effect ( p = .037) for younger participants (under mean age of 34) responding to NAC. This remained significant using OCD severity as a covariate ( p = .044). For those under 34 years of age with less than 17 years of OCD duration, this was also significant ( p = .037). Regression analysis within the NAC treatment group also revealed that duration of OCD presentation was a significant predictor of Yale-Brown Obsessive Compulsive Scale (YBOCS) change at study endpoint ( p = .019), whereas baseline Montgomery–Asberg Depression Rating Scale scores were also a trend-level predictor ( p = .060) of YBOCS change in the NAC group.