William C. Oates was born in Pike County, Alabama to William and Sarah (Sellers) Oates, a poor farming family. At the age of 17, he believed that he had killed a man in a violent brawl and left home for Florida. Oates became a drifter, settling in Texas for a couple of years before returning to Alabama at the urging of his younger brother John, who had been dispatched by the family to locate him. He studied law and passed the bar examination, opening a practice in Abbeville.

Oates joined the Confederate Army in 1861 and eventually became the commander of the 15th Alabama infantry regiment in the spring of 1863. He fought in the Battle of Gettysburg, leading his troops in a series of charges on Little Round Top, where his brother John perished. Oates participated in the battles of Chickamauga, the Wilderness, and Spotsylvania Court House. After transferring to the 48th Alabama, he was wounded near Petersburg, Virginia, losing his right arm and ending his active service.

Oates resumed his law practice in Henry County, Alabama, and served as a delegate to the 1868 Democratic National Convention. From 1870 to 1872, he was a member of the Alabama House of Representatives. In 1880, he was elected to the U.S. House of Representatives, where he served seven consecutive terms. Oates married Sarah Toney of Eufaula on March 28, 1882, and they had one son, William Calvin, Jr., who eventually joined his father in the law practice.

Oates was elected governor of Alabama in 1894 in a bitter campaign. Two years later, he unsuccessfully tried to secure his party's nomination as a candidate for the United States Senate. President William McKinley commissioned Oates as a brigadier general in 1898 and he served in the Spanish American War. He returned to his law practice and speculated in real estate. He tried unsuccessfully to have a monument erected at Gettysburg to his fallen brother and comrades in the old 15th Alabama.

Oates died on September 9, 1910, and was buried in Oakwood Cemetery in Montgomery.