The current generation of dedicated Mixed Reality (MR) devices can be considered as the first generation, which is truly mobile while also being capable of sufficient tracking and rendering. These improvements offer new opportunities for the on-set use of MR devices enabling new ways of using MR. However, these new use cases raise challenges for the design and orchestration of MR applications as well as how these new technologies influence their field of application. In this paper, we present MR On-SeT, a MR occupational health and safety training application, which is based on the experiences of an operational division of a world-wide operating German company. The intended purpose of MR On-SeT is to increase employees’ awareness of potential hazards at industrial workplaces by using it in occupational health and safety training sessions. Since the application is used at various locations throughout the company’s world-wide subsidiaries, we were able to evaluate it through an expert survey with the occupational health and safety managers of seven plants in France, Germany, Japan, and Romania. They reported the condensed experience of around 540 training sessions collected within three months. The purpose of the evaluation was twofold: 1. to understand their perceived attitudes towards the application-in-use, and 2. to collect feedback they received from respondents in training sessions. The results suggest that MR On-SeT can be used to extend current, predominantly theoretical, methods of teaching occupational health and safety at work, which also motivates existing employees to actively engage in the training sessions. Based on the findings, several further design implications are proposed.