The aim of this randomized, double-blinded, placebo-controlled, cross-over designed study was to demonstrate the clinical effect, registered by a survey, of a 10-week period of omega-3 fatty acid supplementation of the diet (1.53 g eicosapentaenoic acid (EPA) and 0.31 g DHA, both per 1000 kcal ME, equivalent to the complete diet) of 16 cats with radiologically documented, naturally occurring osteoarthritis (OA), in comparison with a 10-week period of supplementation with corn oil (0.00 g EPA and 0.00 g DHA, both per 1000 kcal ME). Cats on the fish oil revealed higher activity level (p = 0.07), more walking up and down the stairs (p = 0.07), less stiffness during gait (p = 0.03), more interaction with the owner (p = 0.07) and higher jumps (p = 0.03) compared to those on corn oil supplementation. In conclusion, supplementation of long-chain omega-3 polyunsaturated fatty acids changes the owner's perception of some aspects of behaviour and locomotion in cats with naturally occurring OA.