Objectives: This study aimed to demonstrate the bone mineral density (BMD) and body composition in postmenopausal women with knee osteoarthritis (OA) who underwent surgical treatment, such as total knee arthroplasty, osteotomy, or meniscectomy. Methods: A total of 254 women with OA aged 50 years who underwent surgical treatment were enrolled in this study. We evaluated obesity-related factors, muscle components, and BMD using dual-energy X-ray absorptiometry. Results: No significant differences were noted in the BMD of the hip joint between the symptomatic side of the leg with knee OA and the contralateral side. However, when comparing the BMD of each component, the results indicated a significantly higher BMD in the obesity group based on body mass index (BMI). When defining sarcopenic obesity (SO) using various indicators of obesity (BMI, the estimated visceral adipose tissue area, android/gynoid ratio, and total body fat percentage), the prevalence of SO in the OA group who underwent surgical treatment ranged from 22.0% to 49.6%. Conclusions This study investigated obesity-related factors in patients with advanced knee OA who underwent surgery, revealing a high prevalence of overweight/obese individuals, the presence of SO, and a complex relationship between obesity, body composition, and bone density, highlighting the potential protective effects of weight-bearing on bone health while exploring the impact of sarcopenia on bone density differences in the context of OA. Depending on various definitions of obesity, diverse proportions of SO in patients with OA have been observed, and further detailed research is required to understand its impact on the condition.