Bilbo Baggins owns the One Ring at the beginning of J.R.R. Tolkien’s The Lord of the Rings.
Bilbo found the Ring during his journey in The Hobbit, after taking it from Gollum beneath the Misty Mountains. He initially described it as a useful magic ring and kept its origin secret from most people. By the opening of The Lord of the Rings, he has possessed it for decades.
At his eleventy-first birthday party, Bilbo leaves the Shire and gives the Ring to Frodo. Gandalf later investigates the artifact and confirms that it is the One Ring forged by Sauron. Frodo then accepts the task of carrying it away from the Shire.
The ownership transition can cause confusion because Gollum possessed the Ring before Bilbo, and Frodo carries it for most of the trilogy. The question asks specifically about the beginning of The Lord of the Rings, when Bilbo is still its owner.