Expatriates are important parts of transnational companies’ (TNC) foreign subsidiary staffing programs because TNCs heavily rely on expatriates to achieve coordination/control and knowledge transfer in their subsidiaries (Boyacigiller, 1990; Tan & Mahoney, 2006). Thus, expatriates’ job performance is an important factor directly contributing to the success of a subsidiary. Although research has focused on the effects of various dispositional and situational characteristics, job attitudes, and cross-cultural adjustment on expatriates’ task performance (e.g., managerial behaviors, Black & Porter, 1991; the Big Five, Dalton & Wilson, 2000), it underexplored the effects of these variables on expatriates’ OCB performance. Therefore, I aimed to fill this research gap by exploring the effects of various distal (e.g., Agreeableness, Conscientiousness, POS) and proximal (e.g., job satisfaction, work adjustment, organizational commitment) antecedents on expatriates’ OCB performance. In addition, I investigated the possible mediating effects of expatriate adjustment and job attitudes and moderating effect of perceived similarity of host-county culture (i.e., cultural novelty) on the relationship between expatriates’ OCB performance and its antecedents. In the pilot study, I constructed the Cross-Cultural Work Similarity Scale, which measures the novelty of host-company work settings. In the main study, I explored the direct and indirect (through expatriate adjustment and job attitudes) effects of distal predictors on expatriates’ self- and coworker-rated OCBI (organizational citizenship behavior directed at individuals) and OCBO (organizational citizenship behavior directed at the organization) performance. First, the results showed that Agreeableness and collectivity orientation were important predictors of self-rated OCBI, whereas Conscientiousness was of self-rated OCBO; and interaction adjustment, host country language skills, and Agreeableness were important predictors of coworker-rated OCBI and Conscientiousness was of coworker-rated OCBO performance. Second, mediation analyses based on coworker-rated OCB data and isolated mediation analyses based on self-rated OCB data supported the partial mediation effects of expatriate adjustment. Third, the results did not support the possible moderating effect of cultural novelty. These findings highlighted the relative importance of dispositional characteristics (e.g., Agreeableness, Conscientiousness) and expatriate adjustment in predicting expatriates’ OCB performance. Furthermore, the result supported Ilies, Fulmer, Spitzmuller, and Johnson’s (2009) findings regarding the differential validity of personality predictors of OCBI and OCBO performance.