Interest in citizen participation is increasing generally. Almost all operational research (OR) is engaged with clients, but it is mainly in the areas of Soft and Community OR that wider stakeholder and citizen participation has been a significant focus. It is the involvement of citizens that is the subject of this paper. We surveyed OR literature and compiled a corpus of 62 studies, the earliest from 1970, to systematically characterize the involvement of citizens in OR processes. Our review produced three findings: First, some fields of OR have embraced citizen participation, but this is not yet a major concern outside the field of Community OR. Second, citizen participation in OR processes is often driven by a moral rationale. Third, progress in information and communication technology (ICT) enables broad participation, but traditional processes requiring physical presence can also be participatory. From these insights, we formulate research opportunities for OR. (1) OR may join Community OR's endeavor to engage with and empower citizens who have so far rarely been involved in OR processes. (2) OR may identify benefits and drawbacks of digital OR processes in empirical studies. (3) OR may determine whether involving large numbers of citizens is suitable for the societal scale. (4) OR may research building and maintaining trust. (5) OR may join efforts for data protection of participants. (6) OR may systematically report and reflect on participatory OR processes. (7) OR should continue researching the fair aggregation of individual inputs. Citizen participation in OR is topical and challenging. Pursuing these research opportunities will contribute to OR fulfilling its mandate of better decision-making in close cooperation with all affected stakeholders.