The six-acre (24,000 m²) site at the corner of Lot Street (Queen Street West today) and College Avenue (University Avenue today) was acquired by the Law Society in 1828. At the time, the location was on the northwest edge of the city, which has since grown around the building. It was originally bounded on its north side by Osgoode Street, and on its east side by a street that would eventually be known as Chestnut Street. The former no longer exists, and the latter now stops at Armoury Street as Nathan Phillips Square now lies to the east. The original 21⁄2-storey building was started in 1829 and finished in 1832 from a design by John Ewart and W. W. Baldwin. The structure was named after William Osgoode, the first Chief Justice of Upper Canada (what is now the Canadian province of Ontario).
Between 1838 and 1843, the hall was turned into troop barracks. When the Law Society regained possession in 1844, an expansion was designed by Henry Bowyer Lane; the West Wing and Library were built, with two domes (later removed) over the library to connect the two wings. In 1846 the Law Society entered into an agreement with the government to house the province's Superior Court at the hall. Today, the building is jointly owned by the Law Society and the Government of Ontario.
From 1855 to 1857 the building was refurbished and enlarged again, according to a design by the firm Cumberland and Storm, to accommodate courts with the original 1829 building becoming the east wing. From 1880 to 1891 the building was again expanded twice in order to accommodate the law school.
Despite the expansions, the hall presents a unified design in the late Palladian style. The iron fence surrounding the lawns of Osgoode Hall has become a landmark in itself. Its distinctive iron gates are narrow and restrictive; it is a long-standing myth that they were designed to keep livestock out of the grounds of the hall and in the 1950s, students did attempt to pass a cow through one of the gates (they failed). The gates were likely due to Victorian architectural fashion, rather than wandering cattle.
Two libraries are housed within Osgoode Hall: the Great Library of the Law Society of Upper Canada and a smaller library for judges. The Great Library was designed by Cumberland and Storm (1857–1860) and features an ornate plaster ceiling, cork floors, and etched glass windows. A War Memorial by Frances Norma Loring (1887–1968), sculpted in 1928, was added to the Library in honor of Ontario lawyers and law students killed during the First World War. Behind the Great Library (and accessible through it) is the American Room, designed by Burke and Horwood in 1895, a more intimate room with a spiral staircase. The Toronto Courthouse at 361 University Avenue directly to the north is accessible through a connecting tunnel.