Osse was a Maia servant of Ulmo (the Vala of the Sea). Melkor tried hard to recruit Osse early on - and it seems he nearly succeded."But Uinun, at the prayer of Aule, restrained Osse and brought him before Ulmo; and he was pardoned and returned to his allegiance, to which he has remain faithful." (The Silmarillion p. 30) Uinen was Osse's wife.

Osse was the kind of independantly powerful, toying-with-evil kind of being who would have found the temptation of the One Ring irresistable.