Encompassing a diverse population of developers, non-technical users, organizations, and many other stakeholders, open source software (OSS) development has expanded to broader social movements from the initial product development aims. Ideology, as a coherent system of ideas, offers value commitments and normative implications for any social movement, so does OSS ideology for the open source movement. However, the literature on open source ideology is often fragile, or lacking in empirical evidence. In this paper, we sought to develop a comprehensive empirical theory of ideologies in open source software movement. Following a grounded theory procedure, we collected and analyzed data from 22 semi-structured interviews and 41 video recordings of Open Source Initiative (OSI) board members' public speeches. An empirical theory of OSS ideology emerged in our analysis, with six key categories: membership, norms/values, goals, activities, resources, and positions/group relations; each consists of a number of themes and subthemes. We discussed a subset of carefully selected themes and subthemes in detail based on their theoretical significance. With this ideological lens, we examined the implications and insights into open source development, and shed light on the research into open source as a social-cultural construction in the future.