The National Astronomical Observatory of Chile (Spanish: Observatorio Astronómico Nacional de Chile - OAN) is an astronomical observatory owned and operated by the Department of Astronomy of the University of Chile (UCh). It is located on Cerro Calán, a hill in the commune of Las Condes. The commune is an eastern suburb of Santiago located in Santiago Province of the Santiago Metropolitan Region. OAN was founded in 1852 and became a part of UCh in 1927. The facility on Cerro Calán was completed in 1962.

OAN can be traced back to the Gillis Expedition, a project of the United States Naval Observatory. Led by James Melville Gilliss, it arrived in Chile in 1849 to observe Mars and Venus from the southern hemisphere so as to improve solar parallax. Gilliss and his party set up their equipment on Santa Lucia Hill, a small rise in downtown Santiago. After completing the project in 1852, Gilliss sold the equipment and the buildings that housed it to the government of Chile, which formed OAN at that time.

After two years of operating on Santa Lucia Hill, the director of the new observatory, Carlos Guillermo Moesta, noticed that daytime heating of the dark rock of the hill caused the entire landform to move slightly. As a result of this discovery Moesta decided it would be best to move the observatory elsewhere. A new facility was built in what is now Quinta Normal starting in 1857, and OAN officially moved to the new location in 1862. However, it proved to be one of the foggiest locations in the area. The building is now home to the Aeronautical Technical School of the Civil Aviation Authority of Chile.

In 1908 President Pedro Montt appointed Friedrich Wilhelm Ristenpart of Germany director of OAN. Ristenpart organized another move of the observatory, this time to what is now the suburb of Lo Espejo, south of Santiago. Ristenpart died in 1913, and the subsequent director, Alberto Obrecht, completed the move in 1916. The buildings in Lo Espejo have been torn down.

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