Nowadays, the use of purposive inflows and outflows of knowledge to accelerate internal innovation and to expand the markets for external use of innovation has become essential in business strategies and national policies. In addition, the importance of knowledge and technology diffusion requires better understanding of knowledge networks and national innovation systems (NISs). We want to answer the following research question: what kinds of effects will occur in NISs if different levels of open innovation policies are introduced into NISs? According to our literature review, perfect open innovation policies should have three aspects: knowledge and technology production, distribution and consumption. We built a causal loop diagram and a system dynamics model to simulate the effects of open innovation policies on NISs and apply the results to the case of Cambodia to develop Cambodia's national science and technology master plan.