Oomoto (大本, Ōmoto; lit. "Great Source" or "Great Origin"), also known as Oomoto-kyo (大本教, Ōmoto-kyō), is a religion founded in the 1890s by Deguchi Nao (1836–1918) and Deguchi Onisaburō (1871–1948). Oomoto is typically categorized as a Shinto-based Japanese new religion. The spiritual leaders of the movement have always been women within the Deguchi family, along with Onisaburō as its founding seishi (spiritual teacher). Since 2001, the movement has been guided by its fifth leader, Kurenai Deguchi.

Oomoto's administrative headquarters is in Kameoka, Kyoto (Onisaburo Deguchi's hometown), and its spiritual headquarters is in Ayabe, Kyoto (Nao Deguchi's hometown). Uniquely among Japanese religions, Oomoto makes extensive use of the constructed language Esperanto to promote itself as a world religion. Oomoto has historically engaged in extensive interfaith dialogue with religions such as the Baháʼí Faith, Christianity, and Islam, since a key tenet of Oomoto is that all religions come from the same source (in Japanese: bankyō dōkon (万教同根)).

Oomoto was brutally suppressed by the Japanese government in 1921 and again in 1935, since the government perceived it to be a threat to its authority. After World War II, Oomoto was fully legalized as a registered religious organization. Various other religions have also been founded by former followers of Oomoto, most notably Seicho-No-Ie and the Church of World Messianity.

In 1892, Deguchi Nao, a housewife from the town of Ayabe, Kyoto Prefecture, declared that she had a "spirit dream" during the Japanese New Year. She became possessed (kamigakari) by Ushitora no Konjin (艮の金神) and started to transmit the kami's words. According to the official Oomoto biography of Deguchi, she came from a family which had long been in poverty, and had pawned nearly all of her possessions to feed her children and invalid husband. After 1895, and with a growing number of followers, Deguchi Nao briefly affiliated herself with the Konkōkyō religion until 1897, since she did not yet have government approval for her religious movement.

In 1898, Deguchi Nao met Ueda Kisaburō, who had previous studies in kamigakari (spirit possession). In 1899, they established the Kinmeikai together, which became the Kinmei Reigakkai later in the same year. In 1900, Kisaburō married Nao's fifth daughter Sumiko and adopted the name Deguchi Onisaburō. Oomoto was thus established based on Nao's automatic writings (Ofudesaki) and Onisaburō's spiritual techniques.

Since 1908, the group has taken diverse names — Dai Nihon Shūseikai (大日本修齋會), Taihonkyō (1913), and Kōdō Ōmoto (皇道大本) (1916). Later, the movement changed from Kōdō Ōmoto ("great origin of the imperial way") to just Ōmoto (大本, "great origin") and formed the Shōwa Seinenkai in 1929 and the Shōwa Shinseikai (昭和神聖会) in 1934.

Asano Wasaburō [ja], a teacher at Naval War College (海軍大学校, Kaigun Daigakkō), attracted various intellectuals and high-ranking military officials to the movement in 1916. By 1920, the group had their own newspaper, the Taishō nichinichi shinbun (大正日日新聞), and started to expand overseas. Much of its popularity derived from a method of inducing spirit possession called chinkon kishin (鎮魂帰神), which was most widely practiced from 1916 to 1921. Following a police crackdown, Onisaburō banned chinkon kishin in 1923. Today, in present-day Oomoto, only the chinkon (鎮魂) aspect is practiced as a form of meditation, but not the kishin (帰神) aspect of spirit possession.

In March 1920, the Ōmoto-affiliated magazine Shinrei published an edition of The Protocols of the Elders of Zion in Japanese for the first time.