According to newly released estimates, Oklahoma is inching closer to having a population of 4 million people, and with the next census happening this year, the state is expecting to see more consistent growth.

From April of 2010 to July 2019, the U.S. Census Bureau shows Oklahoma's overall population grew an estimated 5%, adding about 200,000 people over those ten years.

As of July 2019, the census bureau says there are just over 3.9 million people in Oklahoma. 
That 5.5% growth is larger than both Kansas and Arkansas, but smaller than the national average which is a little more than 6 percent, and far behind Texas which saw a 10 year growth of over 15%. 

When it comes to Oklahoma City, the mayors office shows consistent growth over the past ten years as well, growing an estimated 70,000. With that growth, the 2020 census would put the city back in the top 30 largest cities in the country.
The last time Oklahoma City was in the top 30 was in the 2000 census when OKC was at 29th.