We present the VROnSite platform that supports immersive training of first responder units’ on-site squad leaders. Our training platform is fully immersive, entirely untethered to ease use and provides two means of navigation—abstract and natural walking—to simulate stress and exhaustion, two important factors for decision making. With the platform’s capabilities, we close a gap in prior art for first responder training. Our research is closely interlocked with stakeholders from multiple fire brigades to gather early feedback in an iterative design process. In this paper, we present the system’s design rationale, provide insight into the process of training scenario development and present results of a user study with 41 squad leaders from the firefighting domain. Virtual disaster environments with two different navigation types were evaluated using quantitative and qualitative measures. Participants considered our platform highly suitable for training of decision making in complex first responder scenarios and results show the importance of the provided navigation technologies in this context.