This paper is focusing on the following question: What is Operational Research (OR)? We will show that there is not a single and simple answer. Epistemological assumptions and practical traditions define different types of OR. We have identified three: The technical or hard OR, the practical or soft OR, and the critical OR. Following a historical perspective we will present these three schools. Habermas' theory about the three cognitive interests will provide a framework to understand this development. Finally, some final remarks about the future of OR will be outlined.