Harsh parenting and child characteristics such as opposition and aggression have been found to relate to bullying, victimization, and bullying-victimization, yet not all children display equal vulnerability to harsh parenting. The monoamine oxidase A gene ("MAOA"; "low-activity" variant) may be a key vulnerability allele as it relates to aggression on experience of harsh parenting, and opposition in children, and may therefore be associated with children who become bullies and victims. Using multiple-informant data from 4,893 mother-child pairs participating in the Avon Longitudinal Study of Parents and Children (ALSPAC), we found that (a) harsh parenting increased subsequent victimization in boys, and the risk was higher for those with the low-activity allele; that (b) harsh parenting (mother reported) increased bullying, victimization, bullying-victimization (child self-reported) for boys but not for girls, via irritable opposition (mother and teacher reported); but that (c) this indirect effect was not moderated by "MAOA." The results suggest that vulnerable boys who are treated harshly by their parents have increased victimization experiences, whereas irritable opposition appears related to bullying with and without victimization and related to victimization alone.