Organization, time management, and planning (OTMP) skills are behavioral manifestations of executive functioning linked to academic outcomes. Interventions to improve OTMP skills have shown favorable outcomes. The Children's Organizational Skills Scale parent and teacher forms (COSS-P, COSS-T) are widely used for assessing OTMP skills, but there is limited research on this scale. This study examined the factor structure and construct validity of the COSS in a non-clinical and clinical sample of students in grades 2-8 (Study 1; n = 1138 COSS-T; 1155 COSS-P), and teacher-referred students with OTMP deficits in grades 3-5 (Study 2; n = 184). In Study 1, a bifactor model with a general factor and three subfactors demonstrated good fit for the COSS-P (RMSEA = 0.057) and COSS-T (RMSEA = 0.052). This model was replicated in Study 2. The general factor on both versions demonstrated an acceptable to high level of internal reliability. An acceptable degree of reliability was also identified for subfactor 1 (Task Planning). The COSS was shown to be effective in differentiating children with ADHD from a comparison group drawn primarily from the non-clinical sample (AUC = 0.84 COSS-P; 0.85 COSS-T). Sex differences of medium magnitude were identified on the total score of both COSS versions. Correlations between the COSS-P and COSS-T were variable across samples. The findings provide strong evidence for using the total score on the COSS-P and COSS-T as an index of overall OTMP skills deficits. Given variable cross-informant correlations on the COSS, multi-informant assessment of OTMP skills is strongly indicated.