Thomas O'Connor, cattle rancher, was born in County Wexford, Ireland, in 1817 and sailed to Texas with his uncle James Power in 1834. On September 28, 1834, the Mexican government granted him 4,428 acres as a settler in the Power and Hewetson colony. He aided the surveyors in platting Refugio in 1834. The next year he joined the Texas cause against Mexico. After arriving in Goliad on October 10, 1835, he was among the first to reinforce La Bahía by joining the volunteers commanded by Philip Dimmitt. With John O'Brien, another nephew of Power, he was in charge of the oxcart evacuation of San Patricio, Refugio, and Victoria ordered by Dimmitt. O'Connor was one of the signers of the Goliad Declaration of Independence and was later a member of R. J. Calder's company. At age seventeen, he was the youngest man to participate in the battle of San Jacinto.
After the war he returned to Refugio County and resumed his trade of making saddletrees; from the sale of these he made enough money to provide himself with horses. In October 1838 he married Mary Fagan, whose dowry of a few cattle and horses, together with land granted O'Connor as his colonial claim and for his military service, became the nucleus of the vast land and cattle holdings he afterwards owned. O'Connor was one of the group of settlers who drove out the Karankawa Indians after their attack at Kemper's Bluff on the lower Guadalupe River in 1845. He won a contested election for commissioner of Refugio County in 1848. In 1873 he sold his herd for $140,000 and invested in land, which was then inexpensive. He was the first Refugio County rancher to fence with barbed wire. He eventually acquired more than 500,000 fenced acres and 100,000 cattle in Refugio, Aransas, Goliad, San Patricio, McMullen, and La Salle counties. His estate, valued at $4.5 million at the time of his death, was reportedly the largest individual land and cattle holding in Texas. O'Connor was also an incorporator of the Texas and Pacific Railroad Company.
He was more esteemed in his lifetime, however, for his personal worth, sobriety, and integrity than for his wealth. He had no formal education, but his keen mind and good judgment made him stand out as a leader. He helped settle the important question of assessment of cattle for taxes, for example, in P.P. Courts v. Thomas O'Connor, in which the Texas Supreme Court held that the residence and headquarters of ranch property fixed the jurisdiction for assessment, thus doing away with the double assessments of cattle ranging in two or more counties. O'Connor's wife died in 1843, leaving him to raise three young sons, including Dennis M. O'Connor. Many years later he married Helen Shelly. O'Connor died at his ranch near Refugio on October 16, 1887, and was buried in the family burial ground near the headquarters ranch home on the San Antonio River.