Challenges faced in the field of operational diagnosis have grown a deal in the last decade, especially for complex, time-critical systems. In the meantime, although model-based techniques are being used widely to address the design of complex systems, but are not extended to their operation—in particular when dealing with faults: operators can benefit from the use of formal models for system monitoring and diagnostics. We thus propose a methodology to create a new type of Operations Dedicated Model (ODM) from the already existing system design models (functional and dysfunctional), with the use of the Behaviour Tree (BT) formalism. With the assumption that Safety Analysis (SA) models describe dysfunctional aspects of the system—notably via Fault Tree Analysis (FTA), we us Fault Trees (FTs) as an input for the ODM construction. We also demonstrate our proposed approach on a Satellite Ground Station example, and discuss how ODMs can improve systems’ operations.