This chapter deals with the changeability of order management systems (OMS). OMS are here referred to complex commercial off-the-shelf software (CCOTS) used, for example, for enterprise resource planning (ERP). Due to turbulent conditions in the business environment, a permanent need for change is the defining challenge for enterprises. However, far too often the rigidity of today’s CCOTS-OMS does not allow users to implement the intended changes in the business organization. In order to deal with this challenge, a cybernetic model of order management is presented in this chapter. Additionally, a decision oriented software engineering and architectural design for CCOTS-OMS is sketched. The authors are convinced that these approaches enhance the changeability of the development and operation of CCOTS-OMS as well as their co-operation with a business organization.