The purpose of this study was to determine the benefits of a multiple family group (MFG) service delivery model compared with services as usual (SAU) in improving the functioning of youth with oppositional defiant/conduct disorder in families residing in socioeconomically disadvantaged communities. Participants included 320 youth aged 7 to 11 and their families who were referred to participating outpatient clinics. Participants were assigned to the MFG or the SAU condition, with parent report of child oppositional behavior, social competence, and level of youth impairment as primary outcomes at post-treatment. Family engagement to MFG was measured by attendance to each group session. Caregivers of youth in the MFG service delivery model condition reported significant improvement in youth oppositional behavior and social competence compared with youth in the SAU condition. Impairment improved over time for both groups with no difference between treatment conditions. The MFG led to greater percentage of youth with clinically significant improvements in oppositional behavior. Attendance to the MFG was high, given the high-risk nature of the study population. The MFG service delivery model offers an efficient and engaging format to implement evidence-based approaches to improving functioning of youth with oppositional defiant and/or conduct disorder in families from socioeconomically disadvantaged communities.