Aggressive antisocial behaviours are the most common reasons why adolescents are referred to mental health clinics. Antisocial behaviours are costly in social and financial terms. The aetiology of aggressive behaviours is unknown but growing evidence suggests it is heritable, and certain genetic variants have been implicated as contributing factors. The purpose of this study was to determine whether genes regulating the hormone oxytocin (OXT) were associated with aggressive antisocial behaviour. The case-control study sample consisted of 160 cases of children displaying extreme, persistent and pervasive aggressive behaviour. This case sample was compared with 160 adult controls. We used polymerase chain reaction (PCR) to determine the genotype for three oxytocin gene (OXT) single nucleotide polymorphisms (SNPs): rs3761248, rs4813625 and rs877172; and five oxytocin receptor gene (OXTR) SNPs: rs6770632, rs11476, rs1042778, rs237902 and rs53576. Genotypic analyses were performed using stata, while differences in haplotypic and allelic frequencies were analysed using Unphased. We also performed within-case analyses (n = 236 aggressive cases) examining genotypic and allelic associations with callous-unemotional (CU) scores (as measured by the psychopathic screening device). OXTR SNPs rs6770632 and rs1042778 may be associated with extreme, persistent and pervasive aggressive behaviours in females and males, respectively. These and haplotype results suggest gender-specific effects of SNPs. No significant differences were detected with respect to CU behaviours. These results may help to elucidate the biochemical pathways associated with aggressive behaviours, which may aid in the development of novel medications.