Objective: To examine the impact of reinforcement on reaction time (RT) and RT variability (RT standard deviation [RTSD]) in preschoolers with ADHD with and without oppositional defiant disorder (ODD), and a typically developing (TD) comparison group. Method: Participants were administered a computerized task consisting of two conditions: simple RT (SRT) and reinforced SRT (SRTr). Data were analyzed using two-way (Group × Condition) mixed ANOVAs and followed-up using pairwise comparisons. Results: RTs were significantly shorter and less variable during the SRTr than the SRT condition. A significant Group x Condition interaction was observed for RTSD (F= 3.38, p < 0.05); post hoc analyses indicated that the RTSD of the ADHD + ODD group was significantly more variable than that of the TD group during the SRT condition (F = 4.81, p < 0.05). However, their RTSD was statistically indistinguishable from the other groups during the SRTr condition. Conclusion: Preschoolers who are oppositional/defiant and hyperactive are the most responsive to feedback/reward.