The Computer-based Orthographic Processing Assessment (COPA) is a newly developed assessment to measure orthographic processing skills, including rapid perception, access, differentiation, correction, and arrangement. In this study, cognitive diagnostic models were used to test if the dimensionality of the COPA conforms to theoretical expectation, evaluate individual items' quality, and examine the validity and the learning sequence of each skill. Results showed that the COPA captures five distinctive operating attributes, but some items could be revised to increase their item quality. Correlations with external variables confirmed that performances on the COPA are more strongly related to literacy outcomes than to oral language outcomes but that the COPA also demonstrates discriminant validity relative to even proximal measures of word reading and spelling. The mastery probabilities and best-fitting hierarchical model indicate that four of the five attributes follow a learning progression that is consistent with information processing theory and that was assumed by developers of the COPA.