BOK Financial Corporation is a US-based financial services holding company in the West South Central States region of the United States with 4,800 employees and operations in ten states, servicing clients around the country. Headquartered in the BOK Tower in downtown Tulsa, Oklahoma, the company began as an energy bank more than 100 years ago during the early-century oil boom. Today, it maintains its strength in the energy sector, but has successfully diversified into a variety of industries, businesses and geographies - all of which provide footing for its steady financial position.

BOK Financial's holdings include BOKF, NA, BOSC, Inc. and the Milestone Group. BOKF, NA operates TransFund, Cavanal Hill Investment Management and seven banking divisions: Bank of Albuquerque, Bank of Arizona, Bank of Arkansas, Bank of Kansas City, Bank of Oklahoma, Bank of Texas, and Colorado State Bank and Trust. BOKF, NA has full-service banks located in eight states.

BOKF has a market value of approximately $3422 million USD as April 7, 2016.

BOK Financial traces its roots to the Exchange National Bank of Tulsa, which was formed in 1910 when four men bought the remains of the failed Farmers National Bank. One of the bank's major investors was Harry F. Sinclair, who became the bank's president.

In 1917, Exchange National Bank of Tulsa 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.

During the Great Depression, several Tulsa oil men, including William G. Skelly and J.A. Chapman, pumped $18 million of their own wealth into the bank to keep it afloat. As a result, it was a major player in the early 1930s oil boom. In 1933, it was reorganized as the National Bank of Tulsa (NBT).

In 1975, NBT changed its name to Bank of Oklahoma (BOK). The following year, BOK began moving its headquarters employees into the new BOK Tower.

In 1986, the Federal Deposit Insurance Corporation granted Bank of Oklahoma its Open Bank Assistance package and it was bought in 1990 by Tulsa businessman, George Kaiser. At the time, it was a $2 billion bank with 20 branches in Oklahoma.

Under Kaiser's ownership, BOK began an aggressive expansion effort with the strategy to locate in growing markets near Oklahoma. Currently BOK Financial operates banks in 8 states including Oklahoma, New Mexico, Arizona, Arkansas, Texas, Colorado, Kansas and Missouri.

In 2007, BOK Financial continued their aggressive push, expanding market share in the Dallas-Fort Worth area. Currently, the Bank of Texas is the 8th largest bank based in Texas with assets of 3.7 billion.

On August 17, 2012, BOK Financial acquired Milestone Group. The acquisition is the first since 2007.

In March, 2014, BOK Financial announced the acquisition of MBM Advisors, Inc. of Houston, Texas.

On December 8, 2015, BOK Financial announced the purchase of MBT Bancshares; the parent company of Missouri Bank and Trust of Kansas City, informally known as "mobank."

In 2005, BOK Financial acquired the naming rights for the BOK Center, for a price of $11 million USD.