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
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"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.