Oklahoma State University–Stillwater (officially Oklahoma State University; informally Oklahoma State, OK State, OSU) is a public land-grant research university in Stillwater, Oklahoma. OSU was founded in 1890 under the Morrill Act. Originally known as Oklahoma Agricultural and Mechanical College (Oklahoma A&M), it is the flagship institution of the Oklahoma State University System that holds more than 35,000 students across its five campuses with an annual budget of $1.5 billion. The main campus enrollment for the fall 2019 semester was 24,071, with 20,024 undergraduates and 4,017 graduate students. OSU is classified among "R1: Doctoral Universities – Very high research activity". According to the National Science Foundation, OSU spent $198.8 million on research and development in 2021.

The Oklahoma State Cowboys and Cowgirls have won 52 national championships, which ranks fourth in most NCAA team national championships after Stanford University, University of California, Los Angeles, and University of Southern California . The Oklahoma State Cowboys wrestling is the most successful NCAA Division I program of all time in any sport. As of 2021, Oklahoma State students and alumni have won 34 Olympic medals (21 gold, 5 silver, and 8 bronze). The university has produced 29 Goldwater Scholars, 18 Truman Scholars, 18 Udall Scholars, and 48 Fulbright Scholars, astronauts, and a billionaire. OSU is ranked top 10 nationally and top 100 in the world for contributions to United Nations Sustainable Development Goals.

Students spend part of the fall semester preparing for OSU's Homecoming celebration, begun in 1913, which draws more than 40,000 alumni and over 70,000 participants each year to campus and is billed by the university as "America's Greatest Homecoming Celebration." The Oklahoma State University alumni network exceeds 250,000 graduates.