Orrock originally trained and qualified as a dentist in his home town of Edinburgh. He then moved to Nottingham where he set up his practice and later worked at the Nottingham School of Design, and began to buy some works of art. Orrock pursed a career as a full-time artist following a move to London in around 1866. He then became an associate, and later a full member of, the Royal Institution in 1871 and 1875 respectively.
As a watercolourist and landscape artist, he was hugely influenced by David Cox, which can be seen in both his style and his subject matter. He was a passionate advocate of British art, and following his move to London, Orrock became a major collector. He primarily collected English 18th and 19th-century pictures and watercolours, 18th-century furniture, and 17th and 18th-century Chinese porcelain.
William Hesketh Lever (1851 - 1925) turned to Orrock for advice and encouragement in building his own collection of British art. Lever bought Orrock's entire collection on three occasions, in 1904, 1910 and 1912. These collections included a vast number of paintings, watercolours, drawings, furniture and porcelain.
Orrock campaigned passionately for a national gallery of British art to be established. He believed that British fine and decorative arts were neglected by the artistic establishment of the time. Lever was influenced by Orrock's feelings and began collecting in a more focussed way as a result.
Orrock decorated the rooms in his own house in many different period styles, which subsequently became a popular tourist destination when he opened his house to the public.
It was later confirmed that many of the works in Orrock’s collection (including two watercolours by David Cox; later purchased by Lever) were forgeries. It seems likely now that Orrock forged works that he could sell on to Lever and others.