The concept of organizational commitment has become a hot topic in the literature of industrial psychology and organizational behavior in the past decade. The main purpose of this study was to investigate the relationships of employee characteristic (age, marital status, job tenure, educational level and job level), Perceived organizational support, psychological capital and job satisfaction to organizational commitment of personnel working in oil industry. The four general hypotheses were developed and data was collected from 234 personnel who were randomly chosen from various departments of Imam Khomeini Shazand Oil Refining Company. The instrumentation of the study was common demographic questions and other variables measured by a paper-and-pencil questionnaire based on 5-point Likert item from "strongly disagree" to "strongly agree". Collected data were analyzed via regression by SPSS software. Findings indicated perceived organizational support, psychological capital and job satisfaction significantly predicted affective organizational commitment, but employee's background was not a satisfactory predictor of affective commitment for personnel working in oil industry. Findings suggest organizational managements and psychologists need to consider the importance of psychological factors when designing programs for increasing the organizational commitment of the workers.