The appearance of O-legs is that when both lower limbs stand naturally, the inner ankles of the feet can be close together but the knees cannot be close together, and the appearance is like the letter “O”, which can be corrected and treated. O-leg is a deformity known medically as inversion of the knee. Common causes include malnutrition, early standing, bone fracture, etc., which can lead to imbalance of muscles and ligaments in both legs, and it is difficult to support the lower limbs when standing naturally, and the knee joints are separated, so that the gap formed between the two legs looks like the letter “O”. Long-term untreated O-legs can lead to deformity of the knee joints and muscle disuse and atrophy and other symptoms. The earlier the O-shaped leg is treated, the more favorable the recovery. Treatments include nutritional supplementation, orthopedic shoes or insoles, manipulation and surgery, etc. Therefore, it is recommended to consult a doctor as early as possible to avoid missing the best recovery period.