The first domain registered under the .org top-level domain was mitre.org. The MITRE Corporation registered it in July 1985, making it one of the earliest registered generic top-level domains on the internet.
The .org domain was created in January 1985 as one of the original top-level domains, alongside .com, .edu, .gov, .mil, and .net. It was intended for organizations that did not fit the commercial, educational, governmental, or military categories. MITRE, a nonprofit organization involved in systems engineering and technology research, became the first registrant.
The answer is sometimes confused with symbolics.com, which was the first .com domain, registered earlier in 1985. Harvard.edu is associated with the early history of .edu, while think.com was an early .com registration. The .org registry was later operated by the Public Interest Registry, which assumed responsibility in 2003, but the original first registration remained mitre.org.