Oklahoma was first settled when the United States government consigned the Five Civilized Tribes to the eastern half of what was then a territory. The government promised the tribes that this land would remain theirs forever. The Five Civilized Tribes are the Cherokee, Creek, Seminole, Chickasaw and Choctaw Nations.
The law the United States Congress passed to do this was called the Indian Removal Act. This was in 1830. The Trail of Tears took place after the Treaty of New Echota in 1835.
President Andrew Jackson ignored a Supreme Court ruling when he forced the Cherokees from their homes and sent them on a forced march of almost 2,000 miles. The ruling, which resulted from a court challenge filed by the Cherokees, said he could not remove the Cherokees from their lands. President Jackson ignored the Court and used the military to moved most of the Cherokee Nation to Oklahoma Territory.
A few Cherokees managed to escape this forced removal. They still live in the area of their ancestral lands. My Cherokee ancestors were among these people who evaded removal.
Oklahoma Territory was first opened to white settlement in the first Land Run of 1889. The land run opened central Oklahoma for settlement. My great-grandparents participated in this run, but did not get land. They ended up with land later in Southwest Oklahoma, outside of what is now Lawton.
This is a poster from the first Fourth of July after the run.