Born in Tartu to Tõnis and Marie Oinas (née Litter), he was the eldest of five siblings.

[1] His sister Emma Elisabet (nicknamed "Elo") would marry writer and literary critic Friedebert Tuglas.

[3] In 1907, he enrolled at Peter the Great St. Petersburg Polytechnic University, studying economics, and graduating in 1915.

[1][4] Oinas was a member of the Estonian Provincial Assembly, serving from 26 November 1918 and replacing Hugo Reiman.

Oinas then sat in each of the four sessions of the Riigikogu which followed (in the National Council during the last) before the dissolution of that institution in 1940 following the Soviet annexation of Estonia.