Okamoto was born in Sumoto on Awaji Island in 1888. He and his family moved to Tokyo in 1892 for his father's promotion to the vice-president of Miyako Shimbun.[1] When in elementary school, Okamoto encountered hand fans with beautiful paintings which fascinated him and motivated him to study painting.[2]
In 1906, he was apprenticed to Seiki Kuroda to study yōga at the age of 18.[1] Among his fellow pupils was Ryūsei Kishida, with whom Okamoto formed an artists' group and named it Fusain Society (Fyūzankai) to promote Post-Impressionism.[3] Enthralled by Vincent van Gogh and Paul Cézanne, they held an exhibition challenging the conservative Bunten in 1912.[1][3][4] It angered Kuroda and brought an end to their mentoring relationship, leading to the split of Fusain Society.[1][5] Nevertheless, Okamoto and Kishida organized a new group together with Shōhachi Kimura and Kōtarō Takamura to give an exhibition of their own paintings in October 1913.[6]
In 1914, upon his marriage, Okamoto moved next door to Kusuyama Masao, a popular theater critic and translator of Western literature.[8][9] Kusuyama helped Okamoto expand his activities to include stage design, and also asked Okamoto to draw illustrations for a series of juvenile novels Mohan Katei Bunko, of which he was the editor-in-chief, in 1915.[1][10]
Okamoto began drawing for Kin no Fune, a magazine of children's literature and songs, in 1919.[1] Knowing Ujō Noguchi through his jobs, Okamoto drew illustrations for Noguchi's works.[10]
In 1922, Okamoto was named chief illustrator for Kodomo no Kuni from its second issue.[1]Kodomo no Kuni was sold at a half yen per copy, relatively expensive compared to rival magazines, but was enough competitive due to its high quality of the pictures.[11] Among the ardent readers were Chihiro Iwasaki and Seiichi Horiuchi, who would become leading illustrators for children in the mid-Shōwa period. Horiuchi admired Okamoto's ability to capture facial expressions.[12]
Kodomo no Kuni was completely different from any other book I had ever read. A picture of beautiful evening primroses looked as if they were whiffling with scent in the dusk ... I fell in love with Okamoto's pictures ...
^ abcdefghij"OKAMOTO Kiichi". Kodomo no kuni. Tokyo: International Library of Children's Literature, National Diet Library. Retrieved 14 September 2013.
^Takesako, Yūko (2001). "モダンの空間". In Takesako, Yūko; Ishiko, Jun; et al. (eds.). 岡本帰一 思い出の名作絵本 (in Japanese). Kawade Shobō Shinsha. pp. 85–86.
^ abKondō, Yū (2011). 洋画家たちの東京 (in Japanese). Sairyūsha. p. 235.
^ abKami, Shōichirō (1994). 日本の童画家たち (in Japanese). Kumon Publishing. pp. 51–54.
^ abcdef岡本帰一. コドモノクニ (in Japanese). Tokyo: International Library of Children's Literature, National Diet Library. Retrieved 14 September 2013.
^"File109 レトロな絵本". 鑑賞マニュアル 美の壺 (in Japanese). Japan Broadcasting Corporation. Retrieved 14 September 2013.
^Kishida, Eriko; Deguchi, Yūkō; Iwaya, Kunio (2009). Corona Books (ed.). 堀内誠一 旅と絵本とデザインと (in Japanese). Tokyo: Heibonsha. p. 28.
^Takesako, Yūko (2009). ちひろの昭和 (in Japanese). Kawade Shobō Shinsha. p. 133.
^Yamada, Miho (17 November 2010). ちひろとちひろが愛した画家たち(PDF). 美術館だより (in Japanese). 171. Chihiro Art Museum Tokyo: 2. ISSN1884-7722. Retrieved 21 September 2013.
^Kami, Shōichirō (1974). 聞き書・日本児童出版美術史 (in Japanese). Tokyo: Taihei Shuppansha. p. 96.