[3] The area that would someday be called Oklahoma County was originally inhabited by members of the indigenous nations of the Southern Plains, but by the 1830s the land would become part of the territory assigned to the Seminoles and Creeks after their removal from their ancestral lands in the Southeastern USA.

[4] As a result of the Reconstruction era treaties signed between the US government and the Seminole and Creek nations in 1866, the land was taken from tribal jurisdiction but not assigned to other tribal governments, which in time led it to be called the Unassigned Lands.

[4] County business initially took place in a building at the intersection of California Avenue and Robinson Street until the construction of the first Oklahoma County Courthouse at 520 West Main Street in the 1900s.

[14] Throughout its population, 12.4% were of German, 12.3% Mexican, 10.1% Irish, 7.9% English, and 7.7% American ancestries according to the 2010 census.

This mirrors the growing Republican trend in Oklahoma since the end of World War II.

In the 2022 Oklahoma gubernatorial election, Democratic candidate Joy Hofmeister received her largest vote share of any county, with 55.0% of the vote and a modest improvement over the 2018 Democratic candidate, Drew Edmonson.

[18] Incumbent Republican Governor Mary Fallin was the last member of her party to win the county with 51.3% in 2014.

[19] Also, in the 2018 United States House of Representatives elections in Oklahoma, Democrat Kendra Horn received 52.3% of the vote in Oklahoma County, which was the only county in the state to vote for a Democratic House candidate, thereby changing parties.

[21][22] In the 2020 United States Presidential election, Republican Donald Trump narrowly carried the county (1% margin), down from 10 points in 2016.