As the Fraser River and Cariboo Gold rushes took off in 1858–61 the Oppenheimers established stores catering to the prospectors and settlers in the Interior of the Colony of British Columbia at Yale, Hope, and Lytton in the Fraser Canyon, Barkerville in the Cariboo, and Fisherville at the Wild Horse Creek goldfield in the East Kootenay.

With such a far-flung business, the Oppenheimers soon realized the importance of improving transportation and joined a group which successfully lobbied the colonial government in 1862 to build the Cariboo Road to Barkerville.

Despite business setbacks in 1866 and 1867, David was running the Oppenheimer Brothers warehouse in Yale by 1868 and was a partner in the firm by 1871.

[4] David Oppenheimer was active in the Yale business community and entertained visiting dignitaries such as Governor-General Lord Dufferin.

After over twenty years of marriage, with no children, David's wife Sarah died on October 15, 1880, and was buried in the Jewish Cemetery on Cedar Hill Road in Victoria, British Columbia.

During his four one-year terms as mayor, many city services were established: the fire department, a ferry across Burrard Inlet, the streetcar system and a water connection from the Capilano River.

He promoted the British Columbia mining industry by publishing a pamphlet in England and the United States, as well as sending product samples to eastern Canada.

As a philanthropist, David Oppenheimer donated land to the city for parks and helped found charities such as the Alexandra Orphanage and the Vancouver branch of the YMCA.

The Oppenheimer family also offered land to the Vancouver Jewish community for a synagogue, the Congregation B'nai Yehuda, which was located at Pender and Heatley Streets in the Strathcona neighbourhood, then the focus of the city's Jewish community (soon after renamed Congregation Schara Tzedeck and since relocated to Oak Street).

His monument at the entrance of Stanley Park, in Vancouver, was built with public donations and dedicated on December 14, 1911.