Background To improve quality of care for patients with hip and knee osteoarthritis (OA), general practitioners (GPs) and physiotherapists (PTs) in a Norwegian municipality initiated an intervention. The intervention aimed to increase provision of core OA treatment (information, exercise, and weight control) prior to referral for surgery, rational use of imaging for assessing OA and improve communication between healthcare professionals. This study assessed the effectiveness of this intervention. Methods Forty-eight PTs and one hundred one GPs were invited to the intervention that included two interactive workshops outlining best practice and an accompanying template for PT discharge reports. Using interrupted time series research design, the study period was divided into three: pre-implementation, transition (implementation) and post-implementation. Comparing the change between pre- and post-implementation, the primary outcome was patient-reported quality of OA care measured with the OsteoArthritis Quality Indicator questionnaire. Secondary outcomes were number of PT discharge reports, information included in GP referral letters to orthopaedic surgeon, the proportion of GP referral letters indicating use of core treatment, and the use of imaging within OA assessment. Analyses involved linear mixed and logistic regression models. Results The PT workshop had 30 attendees, and 31 PTs and 33 GPs attended the multidisciplinary workshop. Two hundred eight and one hundred twenty-five patients completed the questionnaire during pre- and post-implementation, respectively. The adjusted model showed a small, statistically non-significant, increase in mean total score for quality of OA care (mean change = 4.96, 95% CI -0.18, 10.12, p:0.057), which was mainly related to items on OA core treatment. Patients had higher odds of reporting receipt of information on treatment alternatives (odds ratio (OR) 1.9, 95% CI 1.08, 3.24) and on self-management (OR 2.4, 95% CI 1.33, 4.32) in the post-implementation phase. There was a small, statistically non-significant, increase in the proportion of GP referral letters indicating prior use of core treatment modalities. There were negligible changes in the number of PT discharge reports, in the information included in the GP referral letters, and in the use of imaging for OA assessment. Conclusion This study suggests that a primary care intervention including two inter-active workshops can shift the quality of care towards best practice recommendations. Trial registration ClinicalTrials.gov: NCT02876120.