Disaster response is a highly collaborative and critical process that requires the involvement of multiple government agencies and emergency responders (ERs) ideally working together under a unified command to enable a rapid and effective operational response. Following the 9/11 and 11/13 terrorist attacks, and the devastation of hurricanes Katrina and Rita, it is apparent that inadequate communication and a lack of interoperability among the ERs engaged on-site can adversely affect disaster response efforts. Within this context, we present a scenario-based terrorism case study to highlight the challenges of operational disaster command and response. In this work, which is based on the French emergency response doctrine, we introduce a semantics-based common operational command system that is designed to guarantee an efficient information flow among ERs. In particular, our focus is on offering to all ERs a real-time operational picture of the situation in order to enable multi-level coordination among firefighters, police, gendarmerie, healthcare units, public authorities, and other stakeholders. Our approach consolidates information in order to promote timely sharing among ERs. The proposed system is based on an ontology that has been developed to represent the different types of knowledge on the part of ERs, providing a shared vocabulary that covers a variety of interoperability concerns arising for example because data are collected in different formats, because the different functions of different stakeholders are not taken into account, and because there are failures of coordination among different groups of emergency responders.