The key actors in information systems development (ISD) up-to-now have been considered to be composed of a bilateral structure, user and developer. However, in the case of outsourced ISD(OISD), the key actors are deemed to form a triangular structure comprising users, internal IS staff, and an external vendor. This study examines the relationships of the three key actors' competencies (vendor competence, IS staff competence, IT-related user competence) or the partnerships between them (internal partnership, external partnership) and the success of OISD projects. The OISD success is measured by process control and system quality. This study also examines the moderating effect on the above relationships by the structuredness of the task developed into a system. Data was collected from 67 OISD projects in Korean business firms. These results clarify the importance of internal IS staff in OISD projects by showing that the competence of internal IS staff and their partnership together with users or external vendors are critical factors in determining OISD success. This study also discusses the relationships between key actors' competencies and OISD success, and the partnerships between them and OISD success. The results show that these relationships may vary according to some contingency factors by showing that vendor competence and external partnership are more important in the case of less structured target tasks. In addition, several implications are presented, and future directions are suggested.