BOK Financial Corporation — pronounced as letters, "B-O-K" — is a financial services holding company headquartered in Tulsa, Oklahoma. Offering a full complement of retail and commercial banking products and services across the American Midwest and Southwest, the company is one of the 50 largest financial services firms in the U.S., and the largest in Oklahoma. The company's banking subsidiary, BOKF, NA, operates under the brands Bank of Oklahoma, Bank of Texas, and Bank of Albuquerque, and it operates as BOK Financial in Arizona, Arkansas, Colorado, Kansas, and Missouri. It also operates ATM and debit card processor TransFund, Cavanal Hill Investment Management, BOK Financial Securities, BOK Financial Advisors, BOK Financial Insurance, and BOK Financial Asset Management. The company is more than 50% owned by George Kaiser, who acquired the bank in 1991 from the FDIC. Known for its energy roots, as of June 30, 2021, 14% of its loan portfolio was to borrowers in the petroleum industry. History BOK Financial was formed in 1910 primarily as a source of capital for the energy industry and broadened its geographic and market reach to become one of the largest and most multifaceted regional financial institutions in the country. The company traces its roots to the Exchange National Bank of Tulsa, which was formed in 1910. In 1917, Exchange National Bank began construction of a headquarters building in Tulsa at Third Street and Boston Avenue. In 1928, the bank constructed a 28-story tower adjacent to the initial building. The combined structure, since renamed the 320 South Boston Building, remained the tallest building in Tulsa until 1967. The bank has continuously operated a branch in the building since then. The bank was rebranded as National Bank of Tulsa in 1933 and in 1975 to Bank of Oklahoma to reflect its statewide approach. Also in 1975, the bank installed the first automated teller machine in Oklahoma. The following year, the company began moving employees into the new BOK Tower and in 1979, the bank reached $1 billion in assets. Five years later, in 1984, the company merged with Fidelity of Oklahoma and reached $3 billion in assets. On June 7, 1991, with the company's assets totaling just under $2 billion, it was acquired by George Kaiser for $61 million in a sale facilitated by the FDIC. Under Kaiser's ownership, BOK Financial developed a strategy to locate in growing markets near Oklahoma. Footprint and family of