Background: Based on the multitude of findings, nutrition is becoming increasingly important in the treatment of attention deficit/hyperactivity disorder (ADHD) in children. One promising approach is the so-called oligoantigenic diet (OD). This intervention involves avoiding certain foods that often trigger intolerances and allergies. Previous studies have shown that around 60% of patients experienced a significant reduction in ADHD symptoms after completing such a diet. The aim of the present study was to further confirm the efficacy of the OD within an analysis focusing on the symptom of impulsivity. Materials and Methods: In the present study, the Parent Rating of the Diagnostic System of Mental Disorders in Children and Adolescents (DISYPS-II FBB-ADHD) questionnaire was used to measure the severity of ADHD symptoms. Of 34 children and adolescents (between 7 and 18 years of age) screened and included in this study, 31 participants completed the 4-week OD diet. Results: The corresponding post-diet analysis showed significant short-term improvements for the DISYPS-II FBB-ADHD total score, compared to the start of the diet. This pattern of results also applied to the respective subscales of the DISYPS-II FBB-ADHD questionnaire. A follow-up evaluation conducted 3.5 years after the intervention with 21 participants suggested that the improvements in ADHD symptoms were maintained over time. Specifically, 66.7% of the participants continued to meet the responder criterion, with particularly notable and lasting reductions in impulsivity. Discussion: These results suggest that the beneficial effects of the oligoantigenic diet followed by identifying and avoiding individual intolerant foods may persist long term, and participants’ dietary habits may have also evolved over the years. Conclusion: The oligoantigenic diet may have long-term therapeutic potential for reducing ADHD symptoms, especially impulsivity, in children and adolescents.