Although the use of models for the design of complex systems is nowadays prevalent, that is often not the case for their operations and maintenance. Complex systems are characterised by high criticality levels as regards to time and safety constraints. The lack of appropriate models for the monitoring of these systems, or the large volume and complexity of the existing ones e.g. system simulators, impose the need for the means and tools provision to the operators, to help them with their diagnostic tasks. The use of formal models for system monitoring and diagnostics can greatly augment the operators' comprehension of the system. To that end, we propose a methodology to create a new type of Operations-Dedicated Model from already existing system design models (functional and dysfunctional), using the Behaviour Trees formalism. With the assumption that Safety Analysis models describe dysfunctional aspects of the systemnotably via Fault Tree Analyses, we us Fault Trees as a direct input for the ODM construction. We demonstrate our proposed approach with an Earth Observation Satellite Ground Station example, and discuss how ODMs can improve systems' operations.