Feline osteoarthritis (OA) diagnosis depends greatly on owner perception and experience of the disease. Owners frequently consider the emergence of OA (delayed and severe) signs as “normal” for an aged cat. Being an incurable, progressive, degenerative process, OA will require life-long treatment. There is a need for a rapid, reliable, and inexpensive diagnostic tool that reveals feline OA pain. A refined scale, the Montreal Instrument for Cat Arthritis Testing, for Use by Veterinarians (MI-CAT(V)) presented remarkable metrological properties (specific, sensitive, and reliable) over its development and validation process. In particular, MI-CAT(V) was responsive to a treatment with firocoxib, a non-steroidal anti-inflammatory drug (NSAID), and discriminated four degrees of OA pain functional severity. Firocoxib presented a clear treatment effect based on the MI-CAT(V) and on other functional objective assessments, including possible dose-response. The NSAID was safe over the three-week daily administration tested period. The cluster repartition offers new perspective for an individualized treatment care.