Background: To test the factor structure of oppositional defiant disorder (ODD) symptoms and to study the relationships between the proposed dimensions and external variables in a community sample of preschool children. Method: A sample of 1,341 3-year-old preschoolers was randomly selected and screened for a double-phase design. In total, 622 families were assessed with a diagnostic semi-structured interview and questionnaires on psychopathology, temperament and executive functioning completed by parents and teachers. Results: Using categorical and dimensional symptoms of ODD it was possible to confirm, cross-informant and cross-method, distinct dimensions for defining the structure of ODD: one made up of irritable and headstrong and the other of negative affect, oppositional behaviour and antagonistic behaviour. Specific associations with DSM-IV disorders were found, and irritable was associated with anxiety disorders, whereas headstrong was associated with disruptive disorders, including aggressive and non-aggressive CD symptoms. Also, negative affect was associated with anxiety disorders and non-aggressive CD symptoms, oppositional behaviour with disruptive disorders and aggressive CD symptoms, and antagonistic behaviours with disruptive disorders and, in boys, with mood disorders. The dimensions correlated with specific scales of psychopathology, temperament and executive functioning. Conclusions: Oppositional defiant disorder is a heterogeneous disorder from preschool age. Different dimensions, with moderate to acceptable reliability and convergent and discriminant validity with other psychological constructs, can be identified early in life. (Contains 5 tables and 1 figure.)