William S. O'Brien

William S. O'Brien (1825 – May 2, 1878) was an American businessman who 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. Together they also established the Bank of Nevada in San Francisco, California.

He is considered to have been one of the 100 wealthiest Americans, having left an enormous fortune. He died of Bright's disease.