Today the advantages of project management have been well proven, but project failure rates still remain high. This suggests exploration of new process models and organization structures to enhance project performance. One important candidate for improvement in project susccess is the project management office (PMO). The present study, based on quantitative approach and survey strategy, examines the project management office and its functions and theire relationship to project success. The statistical population of this research is project-oriented organizations of upstream Iranian oil and gas industries. To accomplish this goal, two questionnaires were developed. The first questionnaire was designed for collecting project office and second questionnaire for project success data collection. These questionnaires were completed in 47 Project Management Offices in 41 organizations. Results show that most project management offices had the supportive role. In half of the cases, provided services for all levels of organizations. Among functions of project management office, “methods and standards” have the highest existence and “training” had the least existence. Although in terms of activity level, consulting organizations, general contractor, design-build contractor, construction manager and employer and in terms of economic sector, private, semi-private and public organizations have a more suitable condition from the perspective of functions, respectively but there was no significant difference among them. Among the seven functions of the project management, office Methods and standards function had the highest correlation to project success.