The tribes have their own laws and that is why not all have legalized the equal marriage after the decision of the US Supreme Court to allow the same-sex marriage across the country.
Osage had decided to hold a referendum on the issue and with 52% approval, the definition of marriage in the tribal legal code will be amended to include same-sex couples.
"I know that for a lot of people it was a controversial issue, but for me, it was not," said Osage legislator Alice Buffalohead, the measure's author, told NBC News.
"To every Native kid just beginning to understand who they are inside, the tribe has made it clear we support and love them," Henry Roanhorse Gray, co-creator of a Facebook group called Osage Citizens for Marriage Equality, added.
The Osage Nation join the Cherokee Nation and the Cheyenne, Arapaho or Shell tribes which also allow equal marriage.