Oklahoma State University–Stillwater (also referred to informally as Oklahoma State, O-K-State, O-State and OSU) is a land-grant, sun-grant, coeducational public research university located in Stillwater, Oklahoma, USA. 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. Official enrollment for the fall 2010 semester system-wide was 35,073, with 21,149 students enrolled at OSU-Stillwater. Unofficial enrollment shows the Freshman class of 2011 to be the largest on record, but the official number has yet to be released. OSU is classified by the Carnegie Foundation as a research university with high research activity.

Oklahoma State University is listed by the Princeton Review as one of 120 “Best Western Colleges” for 2011, and as one of 50 "Best Value Colleges – Public" for 2010. It is ranked by U.S. News & World Report No. 66 among "Top Public Schools:National Universities" and No. 132 among all National Universities for 2011. For 2009–10, Kiplinger listed OSU among its "100 Best Values in Public Education," with an in-state ranking of No. 91 and an out-of-state ranking of No. 93.

The Oklahoma State Cowboys' athletic heritage includes 50 national championships, a total greater than all but three NCAA Division I schools in the United States, and first in the Big 12 Conference. OSU's Homecoming celebration, begun in 1913, 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."