Inclusion Criteria:
1. Ability to speak and read English sufficiently to complete study procedures and provide informed consent.
2. Current diagnosis of knee osteoarthritis (KOA) in one or both knees.
3. Average knee pain intensity of ≥ 4 on a 0-10 numeric rating scale during the previous week.
4. KOA-related pain duration of ≥ 6 months.
5. No prior formal mindfulness training or regular mindfulness meditation practice, defined as no previous participation in structured programs such as MBSR or MORE.
6. No prior use of topical diclofenac or oral diclofenac products.
7. Willingness to refrain from taking aspirin or other NSAIDs during the study's treatment period.
8. Willingness to comply with all study procedures, including randomization to either the mindfulness or oral diclofenac arm.
Exclusion Criteria:
1. Known pregnancy.
2. Current use of aspirin or other NSAID medications.
3. Known hypersensitivity, allergy, or clinically significant adverse reaction to NSAIDs or diclofenac.
4. Laboratory abnormalities suggesting increased bleeding risk or other clinically significant hematologic instability, including but not limited to clinically significant anemia, thrombocytopenia, markedly abnormal white blood cell count, or other CBC findings that, in the judgment of the study clinician, suggest the participant should not begin oral diclofenac.
5. Current medications that, in the judgment of the study clinician, may increase risk with oral diclofenac use or interfere with safe participation. This includes, but is not limited to, anticoagulants, digoxin, or other clinically significant medication interactions.
6. Medical conditions that are contraindicated for oral diclofenac use or that may substantially increase risk based on FDA labeling and black box warnings, including clinically significant cardiovascular disease or elevated cardiovascular risk, such as history of myocardial infarction, stroke, serious heart disease, coronary artery bypass graft surgery, uncontrolled hypertension, or other conditions that may increase risk of cardiovascular thrombotic events; and clinically significant gastrointestinal disease or elevated gastrointestinal bleeding risk, such as active or recent peptic ulcer disease, history of gastrointestinal bleeding or perforation, or other conditions that may increase risk of serious gastrointestinal bleeding, ulceration, or perforation.
7. Laboratory or clinical findings suggesting clinically significant renal or hepatic dysfunction, including but not limited to: estimated GFR \< 60 mL/min/1.73 m²; serum creatinine above the laboratory reference range and judged clinically significant by the study clinician; AST or ALT \> 2 times the upper limit of normal; other clinically significant abnormalities identified during healthcare provider review that may increase risk with NSAID exposure.
8. Severe neurologic or vascular disease affecting the lower extremities.
9. Knee trauma, knee surgery, or intra-articular corticosteroid injection within the previous 3 months.
10. Pain primarily attributable to active cancer or ongoing cancer treatment.
11. Unstable or severe medical or psychiatric illness that, in the judgment of the investigators, would interfere with safe participation, study adherence, or interpretation of results.
