Ohio University (Ohio or OU) is a public research university with its main campus in Athens, Ohio, United States. The university was first conceived in the 1787 contract between the Board of Treasury of the United States and the Ohio Company of Associates, which set aside the College Lands to support a university, and subsequently approved by the territorial legislature in 1802 and the Ohio General Assembly in 1804. The university opened for students in 1809, and was the first university to be established in the former Northwest Territory.

Ohio University comprises nine campuses, nine undergraduate colleges, a graduate college, a college of medicine, and a public affairs school. It offers more than 250 areas of undergraduate study as well as certificates, master's, and doctoral degrees. It is a member of the University System of Ohio. The university is accredited by the Higher Learning Commission and classified among "R1: Doctoral Universities – Very high research activity". As of fall 2020, the university's total enrollment at Athens was slightly more than 18,000, while the all-campus enrollment was just over 30,000.

Ohio's intercollegiate athletic teams are known as the Bobcats and compete in the National Collegiate Athletic Association (NCAA) at the Division I level as charter members of the Mid-American Conference. Ohio football has participated in 16 bowl games through the 2023 season. The men's basketball team has made 14 appearances in the NCAA Division I basketball tournament, with their most recent appearance in 2021.

A university in the Ohio Country was first envisioned by Manasseh Cutler, credited as the school's founder along with Revolutionary War Brigadier General Rufus Putnam. In addition to being instrumental in its founding, Putnam was also an original trustee of the university. Putnam Hall there is named for him. Cutler had served as a chaplain in George Washington's Continental Army. The institution's first name was American Western University. In 1797, settlers from Marietta traveled downstream on the Ohio River and up the Hocking River to establish a location for the school in what the Congress designated as the College Lands, founding Athens due to its location directly between the original capital of Chillicothe and Marietta. In 1802, approval was granted by the territorial government for the establishment of the American Western University, but the school was not operated under that name. Ohio University was recognized by the new state on February 18, 1804, with its charter being certified by the Ohio General Assembly. This last approval happened eleven months after Ohio was admitted to the Union. The first three students enrolled in 1809. The first two bachelor's degrees were granted in 1815.

Ohio University was closed between 1843 and 1848. Women were first admitted to the university in 1868. In 1874, the Ohio General Assembly created the new Ohio State University in Columbus as a land grant school upon passage of the Morrill Act of 1862. At that time some representatives proposed that both Ohio University and Miami University be demoted to preparatory schools. In 1880, it was instead suggested that Ohio and Miami be merged directly with Ohio State, but the 1896 Sleeper Bill, introduced by Athenian David L. Sleeper, the speaker of the Ohio House of Representatives, provided annual support for the university; this set the precedent for continuing state support of Ohio University. A second challenge was defeated in 1906.

The 20th century saw dramatic growth in student enrollment, academic offerings, and research facilities. Between 1955 and 1970, undergraduate enrollment tripled from 7,000 to 20,000. During this era, the campus grew, with the construction of 25 new dormitories located on two new residential college greens, with radio and television stations, research and classroom facilities, and the construction of the 13,000-seat Convocation Center arena. Ohio University ranks among the top 25 largest residential college campuses in the United States.^{[citation needed]}

Ohio restructured its two colleges into five in 1935, establishing the colleges of Commerce, Fine Arts, and Applied Science in addition to the existing colleges of Arts & Sciences and Education. The graduate college was created in 1936, and the first PhD program was initiated in 1956 in chemistry. Starting mid–century, the university also began to establish regional campuses throughout southeast Ohio. The first, Ohio University – Chillicothe, was opened in 1946 to help eliminate post-World War II overcrowding on the university's main campus. The school began with 281 students, 70 percent of which were armed services veterans. Later campuses would come in 1946 at Zanesville, 1956 in Ironton and Lancaster, 1957 in St. Clairsville, and formerly Proctorville in 2006.

In 1964, U.S. President Lyndon B. Johnson publicly referenced his Great Society initiative for the first time on the College Green, giving the university exposure across America and internationally. On May 4, 1970, the Ohio National Guard was ordered to open fire on students demonstrating against the Vietnam War at Kent State University, killing 4 and wounding 9. At the same time, there were sit-ins and anti-war riots at Ohio University, even more intense than those of Kent State. This was partly due to the administration's refusal to close the university; instead of going home, many students from other Ohio universities that did close came to Athens to protest further. When the Ohio National Guard was called in to Athens, there was a 3-hour battle at the Baker Center, resulting in 23 injured and 54 arrested students. On May 15, the campus was closed.