Introducing mixed reality (MR) into safety-critical environment like oil refinery is difficult, since the environment and organization lays demanding restrictions for the application. In order to develop usable and safe MR application, we need to study the context of use and derive user requirements from it. This paper describes the user requirements for an MR based oil refinery training tool. The application is aimed to train employees of a specific process unit in the refinery. Training is currently done mainly in a classroom and on-site only when the process is closed down. On-site training is necessary, but expensive and rarely possible. The use of mixed reality offers a way to train employees on-site while the process is running. Users can virtually see “inside” the columns and can modify virtually the process.. CR Categories: H.5.1 [Multimedia Information Systems]: Augmented Realities; H5.2 [User Interfaces]: User-centered design.