Inclusion Criteria:  Documented diagnosis of OA of the knee, meeting American College of Rheumatology (ACR) criteria for classification of idiopathic (primary) OA for at least 6 months prior to Screening Radiologic evidence of OA of the knee of grade 2 (mild) or grade 3 (moderate) according to Kellgren and Lawrence Radiographic Grading Worst daily pain (within 24 hours prior to Screening and Baseline) in the index knee between 4 and 8 on the 11-point pain NRS On stable analgesic therapy  At Baseline:  - Development of a flare of pain following washout of stable analgesic (NSAID and/or acetaminophen) therapy  Exclusion Criteria:  Radiologic evidence of severe OA of the knee (Kellgren and Lawrence grade 4) Secondary OA of the index knee Any other arthritis, included but not limited to rheumatoid arthritis, psoriatic arthritis, etc. History of pseudo-gout or inflammatory flare-ups History of severe neurological conditions Any other chronic pain conditions (e.g., back pain) or disabling conditions affecting the joints Patients who are non-ambulatory or require the use of crutches or a walker, or started using a cane within 30 days prior to Screening Any history of major surgery to the index knee, minor knee surgery, or injury to the index knee within 1 year prior to Screening Knee arthroscopy (index knee) within 3 months prior to Screening Planned or candidate for knee replacement or knee reconstruction surgery Received intra-articular viscosupplementation/hyaluronate, joint lavage, or other invasive therapies to the index knee in the past 90 days Treatment with or need for any of the following: (1) oral or intramuscular corticosteroids within the past 90 days; (2) intra-articular corticosteroid injection into the index knee within the past 90 days, or into any other joint within the past 30 days; (3) current use of topical corticosteroids on the index knee Prior stable therapy with an opioid analgesic, or anticipated need for opioid analgesic use during the study Use of sedative hypnotic medication, antidepressants with known analgesic effect, antipsychotics, antiepileptics, and anti-Parkinson drugs within the past 14 days Regular use of medication for headaches Anticipated use of any oral or topical NSAID (apart from the study drug) during the study Known sensitivity to the study drug (or any of its ingredients), or NSAIDs, or presence of contraindications, warnings, or precautions  At Baseline:  Worst daily pain (within the past 24 hours) in the contralateral knee assessed as >2 on the 11-point pain NRS scale Non-compliance with the daily diary requirement during the Screening period.