To study the prevalence of subthreshold oppositional defiant disorder (ST ODD)—less than 4 symptoms, but nonetheless an impairing form of oppositional defiant disorder (ODD)—its coexistence with other homotypic externalizing and heterotypical internalizing problems in children and associated impairment, as well as the long-term effect of this condition. A population-based sample of 622 preschoolers (5.0% boys) was followed up from preschool to preadolescence. Parents were interviewed when the children were 3, 6 and 9 years old with the Diagnostic Interview for Preschoolers/Children and Adolescents versions following DSM-5 and the children’s functioning was assessed by trained clinicians. ST ODD diagnosis is highly prevalent (19.4–25.5%), highly comorbid [homo- (1.9–18.4%) and heterotypical (5.8–23.7%)], resulting in functional impairment across child development in a similar way for both genders. ST is also a risk factor condition that predicts the presence of psychological problems and impairment in childhood and preadolescence from preschool age. A broader clinical assessment and intervention similar to that provided full syndrome cases is needed for children presenting subthreshold forms of ODD.