Ōu Main Line

On this section, the Ōu Main Line shares the tracks with the Yamagata Shinkansen. The rail gauge is 1,435 mm (4 ft 8+1⁄2 in) to allow the Yamagata Shinkansen to run on it. The Ōu Main Line is known as the Yamagata Line on this section.

Crossing the Yamagata-Akita border, there is little demand in this section, and all trains except one limited-stop "Rapid" train run as all-stations "Local" trains.

On this section, the Ōu Main Line shares the tracks with the Akita Shinkansen. Because the Ōu Main Line occasionally runs from Akita to Shinjō as a local train, this section contains one standard gauge track and two narrow gauge tracks. Also, the few Komachi trains running on this section have the priority.

Together with the San'in Main Line, Maizuru Line, Obama Line, Hokuriku Main Line(including the IR Ishikawa Railway, Ainokaze Toyama Railway, and the Nihonkai Hisui Line), part of the Shinetsu Main Line(including the Myoko Haneuma Line), Hakushin Line, and the Uetsu Main Line, this section of the Ōu Main Line is one of the express lines and freight lines that make up the Nihonkai Jūkan-sen (Sea of Japan Trans-Japan Line).

• December 1, 1894: Aomori–Hirosaki
• October 21, 1895: Hirosaki–Ikarigaseki
• June 21, 1899: Ikarigaseki–Shirasawa
• November 15, 1899: Shirasawa–Ōdate
• October 7, 1900: Ōdate–Takanosu
• November 1, 1901: Takanosu–Noshiro (present-day Higashi–Noshiro)
• August 1, 1902: Noshiro–Gojōme (present-day Hachirōgata)
• October 21, 1902: Gojōme–Akita
• October 1, 1903: Akita–Wada
• August 21, 1904: Wada–Jingūji
• December 21, 1904: Jingūji–Ōmagari
• June 15, 1905: Ōmagari–Yokote

Webb and Thompson large staff instruments used on the Ōu South Line

• May 15, 1899: Fukushima–Yonezawa
• April 11, 1901: Yonezawa–Yamagata
• August 23, 1901: Yamagata–Tateoka (present-day Murayama)
• October 21, 1901: Tateoka–Ōishida
• July 21, 1902: Ōishida–Funagata
• June 11, 1903: Funagata–Shinjō
• October 21, 1904: Shinjō–Innai
• July 5, 1905: Innai–Yuzawa
• September 14, 1905: Yuzawa–Yokote, completion of Fukushima–Aomori connection

Various sections of the line have been double-tracked since 1963.

The section between Niwasaka and Akaiwa stations proved to be geologically unstable, with one of the original tunnels collapsing in 1910. A realignment involving two new tunnels was opened a year later. Geological instability was suspected as the cause of a derailment on the section in 1948 that killed three crewmen, and another realignment was undertaken when the section was double-tracked in 1968.

Itaya station was originally a reversing station, and was realigned as a through station in conjunction with the gauge conversion work (see below) in 1990.

The Fukushima to Yonezawa section was electrified at 1,500 V DC in 1949, and the Uzen-Chitose–Yamagata section in conjunction with the Senzan Line (also at 1,500 V DC) in 1960. Trials on the Senzan Line subsequently resulted in the adoption of 20 kV AC for all further electrification, and the abovementioned sections were converted to the new standard when the Yonezawa to Yamagata section was electrified in 1968. The Aomori to Akita section was electrified (at 20 kV AC) in 1971, as was the Akita to Uzen-Chitose section in 1975.

Akayu human-powered tramway

Kosaka Railway sulphuric acid train in its final year of operation

Kawabe station with a Konan Railway train bound for Kuroishi in 1997

• Akayu Station: The Yamagata Prefectural Government operated a 2 km 610 mm (2') gauge human powered tramway between 1919 and 1926.
• Oishida Station: The Obabazawa Railway opened a 3 km line to its namesake town in 1916, the line closed in 1970.
• Yuzawa Station: The Ogachi Railway Co. opened a 12 km line to Zentsu, electrified at 600 V DC, between 1928 and 1935. The last 3 km section closed in 1967, the electrification was decommissioned in 1971 and the rest of the line closed in 1973.
• Yokote Station: The Yokote Railway opened a 38 km line to Oikata between 1918 and 1930. Construction commenced on an extension to Maego station on the company's Yuri Kogen Railway Chokai Sanroku Line but was not completed. The 12 km section from Oikata to Niiyama was closed following typhoon damage in 1947, the 7 km section from Niiyama to Tateai closed in 1965 when a bridge was destroyed by floodwaters, and the balance of the line closed in 1971.
• Akita Station: The 762 mm (2'6") gauge Nibetsu Forest Railway, consisting of a 22 km main line and five branches between 1.3 km and 5 km in length (and a 550 m tunnel) operated between 1909 and 1968.
• Hachirogata Station: The Akita Chuo Kotsu operated a 4 km line to Gojome, electrified at 600 V DC, from 1922 until 1969.
• Odate Station: The Kosaka Railway opened a 23 km 762 mm gauge line to its Kosaka Refinery in 1908, together with a 4 km branch from Shigenai to Kizawa the following year. Passenger services ceased on the Kizawa branch in 1926, and it closed in 1951. The 10 km Kosaka to Shigenai section was electrified in 1928, and extended 6 km in 1949, but was decommissioned when the line was converted to 1,067 mm gauge in 1962. Passenger services ended in 1994, and the remaining traffic was sulphuric acid, but following two major derailments the line closed in 2009.

The company also opened a 5 km 762 mm gauge line to the Hanaoka mine in 1914 including a bridge over the Ōu Main Line at Odate, which was converted to 1,067 mm gauge in 1951 to enable ore wagons to be forwarded via JNR trains. Freight services ceased in 1983 and the line closed in 1985.

• Kawabe Station: The 7 km line to Kuroishi was opened in 1912, transferred to the Konan Railway in 1984, the year that freight services ceased, and closed in 1998.