Objectives: The aim of our study was to examine the spectrum, demographics, and mortality rate among adolescents and adults with congenital heart diseases (CHD) in Oman. Methods: Data was collected retrospectively from the Royal Hospital, Muscat, electronic health records for all patients with a diagnosis of CHD aged 13 years and above. Data was analyzed according to the type of CHD and in-hospital mortality was assessed using Kaplan-Meier survival analysis. Results: A total of 600 patients with CHD were identified, among them 145 (24%) were aged 18 years or below. The median age was 24 years. The majority of patients had a simple form of CHD. Atrial and ventricular septal defects together constituted 62.8% of congenital heart diseases. Most patients were clustered in Muscat (32%) and the Batinah regions (31.1%) of Oman. Patients with tetralogy of Fallot and Fontan had shorter survival time than recorded in the published literature. Conclusion: Mostly simple forms of CHD in younger patients was observed. The survival rate was significantly shortened in more complex lesions compared to simple lesions. A national data registry for CHD is needed to address the morbidities and mortality associated with the disease.