Inappropriate medications use (IMU) is a serious issue of global concern that leads to a waste of resources and potentially harms the patients. IMU can usually be identified by extracting information about the patient's conditions and treatments, and comparing them with "medication appropriateness criteria". To enable automation of these criteria, we developed a formal representation for them, which we called Objective Medication Appropriateness Criteria (OMAC). OMAC represents four aspects of the criteria: trigger, rules, action and metadata. Our evaluation showed that OMAC can completely represent explicitly defined medication appropriateness criteria using links to external knowledge sources. OMAC is the first formal representation for medication appropriateness criteria, and will enable development of structured rules for appropriate use of medications that can be implemented using standards for clinical decision support.