Exceptions in safety-critical systems must be addressed during conceptual design and risk analysis. We developed a conceptual model of exceptions, a methodology for eliciting and modeling exceptions, and templates for modeling them in an extension of the Object-Process Methodology (OPM)-a system analysis and design methodology and language that uses a single graphical model for describing systems, including their timing exceptions, which has been shown to be an effective modeling methodology. Using an antibiotics treatment guideline as a case study, we demonstrate the value of our approach in eliciting and modeling exceptions that occur in clinical care systems.