The name of the line refers to two localities: Ômachi 大町 (known as Shinano* Ômachi 信濃大町), and Itoigawa (糸魚川). The two characters 大 and 糸 together are read "Ôito".
The Ôito-sen has an overall length of 105, 4 km. The starting point is the town of Matsumoto (592 m above sea level), located in the Matsumoto Basin in the centre of Nagano Prefecture and surrounded by spectacular mountains. Running northwards towards the Japan Sea the line soon follows the Takasegawa River, arriving at Shinano Ômachi after a run of 35, 1 km. From here on the Ôita-sen crosses the watershed and the pass with its beautiful lakes (Yanaba station is around 822 m high). Then begins the steep descent along the Himekawa River towards the Japan Sea. On the west side of the line peaks of the Hakuba mountain range rise to almost 3000 m. At Minami Otari the electrified section (DC 1500 V) of the Ôita-sen ends, and with it the services of JR East Japan. Minami Otari lies at an altitude of about 525 m, and apart from local services from Matsumoto it is also served by at least one express from Tokyo (Shinjuku) per day.
The railway line this far was built into the Shinano Ômachi region in 1915/16, and from here on past Minami Otari down the valley to Nakatsuchi between 1929 and 1935. Electrification to Minami Otari was completed in 1967. Minami Otari is 70, 1 km from Matsumoto in the south, and 35, 3 km into the mountains from Itoigawa and the Japan Sea in the north.
The section today operated by JR West Japan (i.e. Minami Otari to Itoigawa) is not electrified; the line is characterised by very tight curvature and partly steep gradients as it threads its way along the Himekawa River and through numerous tunnels (the longest measures 3125 m) and many snow and avalanche sheds. As mentioned, as far as Nakatsuchi (31, 3 km from Itoigawa) the line was in use by 1935, but beyond the extremely difficult terrain prevented further development northwards at the time.
From the north at Itoigawa the Ôito-sen took shape southwards as far as Kotaki (13, 6 km, about 133 m above sea level) in 1935. However, the intermediate section from Kotaki to Nakatsuchi (about 462 m above sea level) was only completed in 1957, from when on the Ôito-sen could be traversed in its entire length.
The line between Itoigawa and Minami Otari has been seriously hit several times by natural disaster, landslides, severe flooding and torn-away embankments; from July 1995 to the end of November 1997 no through trains could operate between Hakuba (JR East section) and, at first, Nechi (10 km from Itoigawa), then Kotaki (13, 6 km from Itoigawa). Winter services were interrupted from January to March 2006 and February to April 2012. Services were again suspended June to August 2013 due to damaged river embankments, and the violent earthquake in November 2014 prevented trains to run for over a month.
The 2019 timetable sees 7 stopping trains per day Itoigawa - Minami Otari and back, and two more trains from Itoigawa as far as Hiraiwa.
The future of the line is not secure. As of 2022 discussions about JR West's local lines are taking place, the outcome of which are not yet clear.
* The old province name of Shinano corresponds more or less to present-day Nagano Prefecture.
