Chief Dennis Osadebay was born in Asaba, Delta State to parents of mixed cultural backgrounds. He attended Asaba Government School at Asaba, the Sacred Heart School in Calabar and Hope Waddell Training Institute. He joined the labor force in 1930 as a custom officer working in Lagos, Port Harcourt and Calabar. He subsequently went to England to study Law during the 1940s. It was while studying that he started publishing poetic verses. He was then known as a newspaper poet, as most of his writings were published in the West African Pilot and a few other newspapers. In his writings, Osadebay used both his personal life and public events as inspiration. In Africa Sings, a collection of poems, he delved with themes from a personal point of view, such as a sullen poem written about his twenty fifth birthday and the coming of middle age. However, his best work in the volume, were poems written from an impersonal view point. In his adventurous poem, black man troubles, he used pidgin English to lament the status of black Africans in colonial Africa and injustice in the society. His poems were also notable for faithfully representing modern poetic rhythm.
Osadebay was one of the founding members of the National Council of Nigeria and the Cameroons in 1944. He left the country to read law a few years later. After, completing his studies, he returned to Nigeria and established a law practice in Aba and was also made the legal adviser of the N.C.N.C. In 1951, he contested and won a seat on the Western Region House of Assembly which was dominated by the rival Action Group (AG). He soon became the leader of opposition in the region from 1954-1956 but gave the mantle to Adegoke Adelabu in 1956. After the death of Adegoke Adelabu, he took on his familiar oppositional role in 1958. In 1960, he became the president of the Nigerian Senate and upon the creation of the Mid-Western Region in 1963, became the pioneer premier of the newly created region.