The Virtual Organization Management toolkit (VOM) is a distributed system consisting of several partially independent modules specialized in supporting Virtual Organization (VO) manager during the operation phase of the VO. The task of the whole VOM is to assist the VO manager by monitoring performance of VO, alerting in case of upcoming or impending deviation in a VO performance, and on up-to-date data simulating alternative VO schedules and configuration to optimize the VO future performance. In this paper the architecture of the VOM toolkit is presented as well as a simple use-case.