The 58-year-old justice took her seat with fellow justices when the court convened after a two-week recess.
O'Connor reportedly underwent a mastectomy, removal of the breast. Normally, breast cancer patients receive some treatment such as chemotherapy or radiation to prevent a recurrence.
No details of the operation have been confirmed. But O'Connor said in an earlier statement that the cancer was detected in an early stage and that the prognosis is for a full recovery.
O'Connor had previously issued a statement that she intended to be back at the court today.