William Shoney O'Brien | |
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| Born | c. 1825 |
| Died | May 2, 1878 (aged 52–53) |
| Resting place | Holy Cross Cemetery, Colma (reinterred from San Francisco's Calvary Cemetery) |
| Nationality | American |
| Occupation(s) | Saloonkeeper, stockbroker, partner in Comstock Lode mines |
| Organization(s) | Flood & O'Brien Consolidated Virginia Mining Company Bank of Nevada |
| Known for | Being one of the "Bonanza Kings" |
William Shoney O'Brien (c. 1825 – May 2, 1878) was an Irish-born American businessman. He formed a business partnership with fellow Irishmen James Graham Fair, James C. Flood, and John William Mackay, the Consolidated Virginia Mining Company. The four dealt in mining stocks and operated silver mines on the Comstock Lode, and in 1873 discovered the great orebody known as the "Big Bonanza" in the Consolidated Virginia and California Mine, an orebody more than 1,200 feet deep, which yielded in March of that year as much as $632 per ton, and in 1877 nearly $190,000,000 altogether. The four-way partnership, although formally called "Flood and O'Brien," was more commonly known as the Bonanza firm.