This paper addresses a recent call for an open approach to strategic planning based on the three principles of inclusiveness, transparency, and use of information technology (IT). The focus of the study is to examine how these three principles impact the effectiveness of strategic planning. In two case studies, through post-implementation interviews with 24 stakeholders across various organisational levels, moderating factors impacting the effectiveness of open strategic planning (OSP) have been identified. A grounded theory based approach was employed to analyse the interviews as well as researchers’ observations during both strategic planning projects. The results indicate that a number of moderating factors (such as level of trust among stakeholders, diversity of participants, and the level of IT literacy among participating stakeholders) did impact the relationship between the principles of OSP and strategic planning effectiveness. Our work is related to but distinct from prior research linking OSP and effectiveness outcomes because we consider moderating effects perhaps for the first time. A tentative theory of effectiveness of OSP is presented in eight propositions. Then each proposition is discussed in relation to the existing literature on strategic planning effectiveness. The paper concludes with a number of recommendations for future research and some implications for practice.