The goal of this clinical trial is to understand the patient and clinic visit characteristics that affect health outcomes for individuals with knee osteoarthritis (OA) pain using topical diclofenac gel. The main questions it aims to answer are: * What baseline patient characteristics predict response to topical diclofenac? * Does patient physiology during the study visit predict response to topical diclofenac? * Do study visit characteristics predict response to topical diclofenac? Participants will: * attend 2 study visits to complete study questionnaires and have blood drawn * apply topical diclofenac to their knee for 8 weeks * complete biweekly questions about knee pain and diclofenac use between study visits