Adults with O-legs, i.e. inversion deformity of the knee, can be corrected by osteotomy and orthopedic surgery, while those with severe inversion deformity of the knee can be treated with joint replacement. There are congenital and acquired O-legs in adults. For congenital O-legs, there can be no obvious changes for a long period of time, and observation is the main concern at this time. As the age increases, the inversion deformity worsens and knee discomfort occurs, orthopedic treatment can be taken, such as distal femur osteotomy or proximal tibia osteotomy orthopedic treatment. For adults with secondary O-legs, the main cause is degeneration of the medial cartilage due to knee osteoarthritis. Early treatment can be oral nutrition of cartilage, such as glucosamine capsules, as well as anti-inflammatory and pain-relieving medications such as celecoxib and etoricoxib, in order to delay the inversion deformity caused by degeneration of the cartilage. Regardless of the condition that causes O-legs in adults, when severe inversion deformity is present, knee replacement surgery can be performed to restore lower extremity mechanics and improve O-leg conditions, such as unicondylar replacement and total knee replacement. Adults with O-leg deformity should go to the hospital in time, complete the relevant examinations, and follow the doctor’s instructions to carry out standardized treatment after a clear diagnosis, and should not use their own medication, so as not to delay the condition.