Validated clinical outcome scales are essential tools for the reliable assessment of various health conditions, providing critical measures in both clinical trials and real-world practice. However, proper administration and selection of scales remain challenging, particularly in the context of varied patient conditions and healthcare settings. This paper proposes an outcome scale ontology designed to standardize the use and selection of clinical scales to comprehensively assess patient status in a rehabilitative setting. The ontology incorporates key elements such as measurement goals, information needs, specific subscales, and usage-related factors like the expertise of healthcare professionals, use of devices, and clinical conditions of the patient. We present the ontology within the context of the Fit4MedRob initiative, which seeks to improve the implementation of robotics-assisted rehabilitation in clinical practice. Through a use case related to the Fit4MedRob stroke clinical trial, we demonstrate how the ontology can aid in selecting and administering scales correctly, enhancing the standardization of outcome measurement across diverse rehabilitation settings.