The current study expands previous knowledge by investigating the moderating role of attachment styles in the associations between perceived group cohesion, perceived collective efficacy, and changes in the individual report of organizational citizenship behavior (OCB). Data were collected from 180 employees, who took part in a 9-week organizational training course. Upon start of training, participants completed questionnaires assessing their attachment orientations and their individual-level OCB. Upon conclusion, participants completed questionnaires assessing their perceived group cohesion, perceived group efficacy, and OCB. Using hierarchical linear modeling, both group cohesion and efficacy explained the changes in the individual OCB. In addition, attachment anxiety moderated the associations between group efficacy, group cohesion and the changes in OCB. The results emphasize the joint effects of group-and individual-level variables in predicting OCB in a longitudinal design. Theoretical and practical implications are discussed.