Many neurodevelopmental disorders (NDDs) share common learning and behavioural impairments, as well as features such as dysregulation of the oxytocin hormone. Here, we examined DNA methylation (DNAm) in the 1st intron of the oxytocin receptor gene, "OXTR," in patients with autism spectrum (ASD), attention deficit and hyperactivity (ADHD) and obsessive compulsive (OCD) disorders. DNAm of "OXTR" was assessed for cohorts of ASD (blood), ADHD (saliva), OCD (saliva), which uncovered sex-specific DNAm differences compared to neurotypical, tissue-matched controls. Individuals with ASD or ADHD exhibiting extreme DNAm values had lower IQ and more social problems, respectively, than those with DNAm within normative ranges. This suggests that "OXTR" DNAm patterns are altered across NDDs and may be correlated with common clinical outcomes.