Purpose: Oncology nursing is a special field of practice containing many factors that cause moral distress. The purpose of this study was to explore the sources of moral distress in oncology nurses. Methods: This qualitative phenomenological study was conducted with 14 oncology nurses. The mean interview duration was 30 min. Data were analyzed using qualitative inductive content analysis according to the methods of Corbin and Strauss. Results: Four main themes were identified in the study. The first theme, related to the failure of quality of care, includes the failure to provide holistic care and competence problems (not feeling competent in oncology practice). The second theme includes biomedical ethical issues commonly observed in the field of oncology. The third theme includes treatment and care practices, consisting of futile treatments, lack of regulation for 'do not resuscitate' orders and decisions to limit life-prolonging treatment, limited informational authority of nurses, and problems related to educational practices on the patient. The final theme includes problems arising from the health care system and institution's management and the need for regulation to support ethical decisions. Conclusion: Oncology nurses face ethical problems in providing the quality and continuity of care they desire. It is difficult to manage the problems, especially in the end-of-life period. In order to reduce and eliminate these difficulties, it is recommended to make administrative, institutional, legislative, and systemic arrangements.