16, O'Conor began to study law, and he carried out his learning first in the office of Henry W. Stannard, then with Stephen D. Lemoyne, and finally with Joseph D. Fay. O'Conor attained admission to the bar in 1824, before he had attained the statutory age of 21, and soon won high reputation in his profession. He brought the Forrest divorce case to a successful issue for his client, Mrs. Forrest. Contending against John Van Buren and other eminent counsel, he secured her a liberal alimony. This case brought him more than ever into national reputation. Two silver vases were presented to him in its commemoration: one was the gift of thirty women of New York, the other was presented by sixty members of the bar. These he bequeathed to the New York Law Institute, and they are now preserved in the library of the institute. In the same library are preserved the bound records of his cases and opinions—a unique collection that was made by himself, and also bequeathed in his will to the institute. These fill over 100 volumes. Others of his celebrated private cases were the Slave Jack case in 1835, the Lispenard will case in 1843, the Lemmon slave case in 1856, the Parrish will case in 1862, and the Jumel suit in 1871, involving the title to $6,000,000 in real estate. In 1869 he was elected president of the New York Law Institute. He took a prominent part in the prosecution of Boss Tweed and members of the "Tweed Ring". This proceeding began in 1871, and eventually destroyed the Tweed Ring that was then at the height of its power in New York City. In the original cases he was associated with William M. Evarts, James Emott, and Wheeler H. Peckham. These suits were brought in the attorney general's office, a special branch of which was established for the purpose, and named by him the Bureau of Municipal Correction. In 1875, the court of appeals decided that the cases should have been brought by the city. O'Conor immediately drafted the Civil Remedies Act, which was enacted at the next session of the legislature, and under which new suits were at once begun. Disheartened with the issue of the first cases, O'Conor published an account of them, entitled Peculation Triumphant, being the Record of a Five Years' Campaign against Official Malversation, A.D. 1871-1875 (New York, 1875).