Oppé began collecting works of art after he purchased a work entitled 'Llangollen' by John Sell Cotman in April 1904. From this point onwards, he developed a particular interest in British 18th and 19th century drawings and watercolours. Artists such as Thomas Gainsborough, Alexander Cozens and Francis Towne were well represented in his collection, but his interest in drawings was expansive and, overall, it encompassed five centuries of European art, from the Renaissance to the early 19th century. Oppé frequently published on the artists and works that he collected.
After Oppé's death, the collection was divided between his two children. The European works went to his son, Denys, and the British drawings and watercolours were left to his daughter Armide. Although a few works were sold or gifted to close friends and institutional collections favoured by Oppé, the first large-scale sale of works was in 1996 when the majority of the British collection was acquired by the Tate (via Sotheby's) with support from the Heritage Lottery Fund and the National Art Collections Fund. The collection remains a distinct group of material within Tate's holdings, and is known, as per the wishes of the family, as the Oppé Collection. On 5 December 2006, a significant group of 120 European drawings were sold at auction via Christie's, including highly valued works by Agostino Carracci and Francisco Goya.