Effective treatment of oppositional defiant disorder (ODD) and conduct disorder (CD) is dependent on etiological knowledge about what factors contribute to their development and maintenance. Previous research points to a role for social skills in this process, but findings are mixed, possibly due to methodological factors. A sample drawn from two birth cohorts (n = 1,079, 50.1% girls) in the city of Trondheim, Norway, was assessed biennially from age 4 to 16 years. Social skills were rated by teachers and parents and measures of symptoms of ODD and CD were obtained through diagnostic interviews of parents and children (from age 8). Bidirectional relations between within-person changes in social skills and symptoms of ODD and CD, respectively, were assessed by a random intercept cross-lagged model. Increased parent-reported social skills predicted reduced symptoms of both ODD and CD from preschool to adolescence, while teacher-rated social skills predicted reduced symptoms of ODD from 4 to 14 years but predicted reduced symptoms of CD from 6 to 8 years only. Symptoms of ODD and CD predicted reduced teacher-rated social skills on two occasions (6 to 8 years for CD and 8 to 10 years for ODD), but there were no reciprocal paths involving parent-rated social skills. Findings provide support for including social skills training in the prevention and treatment of symptoms of ODD and CD.