Oathkeeper is a Valyrian steel longsword wielded by Brienne of Tarth.
Oathkeeper is one of two swords (the other being Widow's Wail) that were reforged from the greatsword Ice, an heirloom of House Stark. Tywin Lannister commissionend Tobho Mott to reforge Ice into two swords. Oathkeeper was then given to Ser Jaime Lannister, who was unpleased with the weapon and gave it to Brienne when charging her with finding Sansa Stark.