Sauron is the first bearer of the One Ring in J.R.R. Tolkien’s The Lord of the Rings.
Sauron forged the Ring in the fires of Mount Doom during the Second Age, intending to control the other Rings of Power and dominate their bearers. Because he put much of his own power into it, the Ring was both a weapon and a source of vulnerability: destroying it would permanently defeat his ability to recover his former strength.
Isildur cut the Ring from Sauron’s hand after the defeat of Sauron at the end of the Second Age. Isildur was therefore the next important bearer, but not the first. Later bearers included Sméagol, Bilbo, and Frodo.
The Ring’s history can be difficult to track because Tolkien’s narrative begins long after its forging. The short answer is established by the chronology of the legendarium: its maker and original owner was Sauron.