This chapter presents a Basic Organization Structure (BOS) model for building the large and complex distributed cooperative information system in large mutual networks. It argues that a large and complex cooperative information system and its subsystems in a LAN can be modeled by multi-agent organization and basic organization respectively. With the BOS model, such a cooperative information system can be developed easily and it is more manageable, effectively supporting the complicated cooperative methods under uncertain conditions. BOS is mainly used to support the cooperative problem solving among the coarse-grained, loosely coupled, and groups of semiautonomous agents. The essential characteristics, knowledge representations, and computational models of the BOS model are illuminated in this chapter. As an application example, we use the BOS model to realize the distributed Assumption-based Cooperative Problem Solving (ACPS) in the Distributed Traveling Information Management System prototype.