Help the various Osage members in town:
Retrieving a horse: Mount then dismount a horse will cause them to follow you, move back to the Osage member with the horse following you.
Feathers: Kill a chicken in town for the chicken feather, the brown feather comes from a bird just north of Abilene. Give both feathers back to the Osage member that asked for them.
Buffalo hunt: The Osage member will wait for you outside of the prairie, find him then move to the herd of buffalo inside of the prairie, kill herd.
After the above three are done, go to Chief Bigheart and ask him help. Bigheart will send you to retrieve a telegram from the post office. Go to the post office, ask postmaster Bigheart. The post master will look for a telegram and tell you there is nothing for that name. Attempt to leave the room and his assistant will give you the telegram.
Return to Chief Bigheart and give him the telegram. Bigheart will ask you to take the telegram to Father Shoenmaker. Give the telegram to Father Shoenmaker and he will follow you, return to Chief Bigheart. Lots of talking happens, eventually Bigheart will give you [an Osage Orange flatbow] (https://www.legendhub.org/items/details.html?id=1211).