Oldham County is in the northwestern corner of the Panhandle, bordered on the west by New Mexico, on the north by Hartley County, on the east by Potter County, and on the south by Deaf Smith County. The county's geographic center lies at 35°25' north latitude and 102°35' west longitude; Vega, the seat of government, is thirty miles west of Amarillo. The area was named for Williamson Simpson Oldham, pioneer Texas lawyer and Confederate senator. Oldham County comprises 1,485 square miles of relatively level grassland, broken by the Canadian River and its numerous intermittent tributaries; elevations range from 3,200 to 4,200 feet above sea level. The fine sandy loam and caliche soils in the area support a variety of native grasses as well as mesquite, sage, and shin oaks. Larger trees such as elm, hackberry, cottonwood, and oak grow in the river bottoms in some places. The soils are not generally conducive to farming, so the economy of the county is principally based on ranching. The area receives an average of 19.54 inches of rain per year. Temperatures range from an average minimum temperature of 22° F in January to an average maximum of 92° F in July; the annual growing season lasts 186 days.
Oldham County's history has revolved around the Canadian River, which runs in an east–west direction across the northern part of the county. Archeological investigations, beginning with the 1932 excavations of Saddleback Mesa, have unearthed evidence of the Panhandle Pueblo culture. Petroglyphs and other artifacts attest to the presence of other pre-Columbian peoples. Plains Apaches, followed by the warlike Comanches and Kiowas, found refuge in the breaks of the Canadian. Various Spanish entradas utilized the river as they traveled eastward from New Mexico. Probably both the expedition of Francisco Vázquez de Coronado (1541–44) and the Oñate expedition of 1601 crossed the area. It is fairly certain that Pedro Vial passed through in 1786 and 1788. The Facundo Melgares party came through the county as it searched for Zebulon M. Pike in 1806. Likewise, the ciboleros and Comancheros from northern New Mexico all used the Canadian as a major trade route; indeed, the Atascosa Springs area was a frequent trading ground for Comancheros and their Indian customers. Stephen H. Long, Josiah Gregg, James W. Abert, Randolph B. Marcy, and W. W. Whipple led their pathfinding expeditions along the Canadian valley through the area during the early nineteenth century. Buffalo hunters established temporary camps in the area in the 1870s, and they were soon joined by ranchers and pastores. In 1876 the Texas legislature established Oldham County from the huge original Bexar County, and the county was organized in 1880, with Tascosa as the county seat. Caleb B. (Cape) Willingham became the first sheriff, C. B. Vivian was elected county clerk, and William S. Mabry was made county surveyor. Sixteen unorganized Panhandle counties were attached to Oldham County for administrative purposes. A population of 287 in 1880 made the county the second most populous of the Panhandle area; only Wheeler County, on the east side of the Panhandle, had more residents. The ranching industry of Oldham County began very soon after the Red River War of 1874–75 forced the Comanches and other Plains nomads onto reservations in Indian Territory. Soon after the Indian removal, Casimiro Romero and his fellow pastores from New Mexico established sheep ranches, dotted with stone and adobe plazas, throughout the area, along the Canadian River and its tributaries. As a result Mexican-American settlers outnumbered Anglo-Americans for some time. The situation began to change in 1877, when George W. Littlefield started his LIT Ranch just east of Tascosa. Between 1879 and 1881 W. M. D. Lee and his partners bought out many of the pastores and established the LE and LS ranches, supplanting the sheep with cattle. In 1882 the Capitol Syndicate marked off a large amount of Oldham County lands for use in its famous XIT Ranch. Only the southeastern part of the county fell outside the XIT after that time. Following a certain amount of property exchanging and dislocation within the local ranching industry, other ranches (the LX and the , Frying Pan for instance) occupied Oldham County acreage.
Tascosa, originally called Plaza Atascosa, was an Oldham County village by 1875. As one of only three towns in the Panhandle, it developed a reputation as a rowdy and sometimes violent cowtown. When it became the county seat in 1880, its position as a leading early Panhandle town was strengthened. For decades Tascosa continued to serve as a small trade and administrative center. In 1887 the Fort Worth and Denver City Railway, building from Amarillo to Colorado, crossed the northeastern corner of the county, passing within two miles of Tascosa. Another village, "new" Tascosa, popped up on the road less than two miles from the old site. By 1890 the county had five ranches, more than 30,000 cattle, and 270 residents. Oldham County entered the twentieth century as a ranching area supporting 349 residents and only slightly influenced by the railroads crossing it. By 1900 it had twenty-three ranches, encompassing 578,246 acres; the agricultural census reported 30,226 cattle, but no crops, in the county that year. Crop farmers began to move into the area after 1904, when the Chicago, Rock Island and Gulf Railway laid tracks through the southern part of the county for a line connecting Amarillo to Tucumcari, New Mexico. The new railroad encouraged additional settlement, and a small number of wheat farms were established along the Rock Island right-of-way between 1900 and 1910; the towns of Adrian, Vega, and Wildorado also sprang up along the route. By 1910 Oldham County had eighty-seven farms and ranches and a population of 812. About 1,400 acres were planted in wheat that year, along with fifty acres in corn and 693 acres in sorghum. The economy remained essentially dependent on ranching, however, and 25,000 cattle were reported. As the county developed Tascosa slowly lost population and influence to Vega. By 1915, when a special election moved the county seat to Vega, only fifteen people lived in Tascosa. The county as a whole also lost population during the 1910s, although cropland expanded during that decade. Though only 709 people lived in the county by 1920, more than 7,000 acres were planted in wheat, the county's most important crop; another 65 acres were planted in corn, 1,674 acres in sorghum, and 27 acres in cotton. The area experienced a tremendous expansion of wheat farming during the 1920s; 21,000 acres were planted in wheat by 1925, and 57,000 by 1930. In all, 69,000 acres of crops were harvested in Oldham County that year. The number of farms and ranches rose from 86 to 137 between 1920 and 1930. The population almost doubled during the same period, rising to 1,404 by 1930. A major national highway built through the area in the early 1920s, when U.S. Highway 66 was extended from Amarillo to Tucumcari. The expansion of wheat farming continued during the 1930s during the Great Depression. By 1940, 67,000 acres in the county were planted in wheat. Cattle ranching also expanded significantly during this period; while there were never more than 25,000 cattle reported between 1910 and 1930, by 1940 there were 60,000. The number of farms and ranches also rose by 1940 to 177. Nevertheless, the county lost population during the depression; by 1940 only 1,385 people lived in the area. The town of Tascosa, which had been declining for years, was deserted by 1939, but in June of that year Cal Farley acquired the site, tore down most of the crumbling buildings, and built his Maverick Boys Ranch on the site. By 1950 the county's population had increased to 1,627.
Though oil was discovered in the Oldham County in 1957, significant amounts were not produced there until the early 1970s. County lands yielded 263,000 barrels of oil in 1974, 242,000 barrels in 1978, and 1,558,000 barrels in 1982. Production dropped off in the mid-1980s, in 1990, 325,000 barrels of crude were produced. In 2000, 88,479 barrels of oil and 276,917,000 cubic feet of natural gas were produced in the county. By January 1, 2001, 13,420,373 barrels of oil had been taken from county lands since discovery in 1957. In the early 1980s 97 percent of the county's land was in farms and ranches, and 15 percent of the land was cultivated. In 1980 Oldham County produced $5 million worth of farm crops and $18 million worth of beef cattle; thus 80 percent of the county's agricultural production derived from cattle raising. In 2002 the county had 136 farms and ranches covering 936,390 acres, 86 percent of which were devoted to pasture and 13 percent to crops. In that year farmers and ranchers in the area earned $65,949,000; livestock sales accounted for $63,619,000 of the total. Beef cattle were the county's chief agricultural product; crops included wheat and grain sorghum. Both Interstate Highway 40, which replaced old Route 66 in the 1960s, and U.S. Highway 385 also attracted some dollars to the area.
Partly because of the oil and gas industry, the county's population continued to grow, rising to 1,928 by 1960 and 2,258 by 1970, but stabilized thereafter, as the county reported 2,283 inhabitants in 1980 and 2,278 in 1990. In 2014 the census reported 2,070 people living in the area. About 78.8 percent were Anglo, 3.7 percent African American, and 14.9 percent Hispanic. The voters of Oldham County supported the Democratic candidate in virtually every presidential election between 1884 and 1964; the only exceptions occurred in 1928, when they supported Herbert Hoover over Al Smith, and in 1952, when they supported Dwight D. Eisenhower over Adlai Stevenson. In elections from 1968 through 2004, however, county voters backed Republican candidates, except in 1976, when Democrat Jimmy Carter won in the county. Communities in Oldham County include Vega (population, 902), the county seat and ranch trade center; Adrian (159); and Wildorado (210). The county also continues to be the home of Cal Farley's Boys Ranch.