Ossë (from the Valarin Ošošai, Oššai) was a Maia of the Sea associated with Ulmo. He guarded the waters around Middle-earth and islands, and did not go into the deep places of the Sea. Uinen was his spouse, and he was a friend of Círdan the Shipwright.
In the early days of Arda, Ossë turned to Melkor for a time, but Uinen brought him back to the loyalty of the Ulmo. However, Ossë's wild and wilful nature was still evident; he delighted in and enjoyed making dangerous Sea storms.
After the Valar pronounced the Doom of the Noldor, Ossë helped guard the shores of Beleriand to keep the ships of the Noldor from reaching Valinor. His storms had a part in wrecking all the ships that they sent, and only one Elf from among their crews survived, by the will of Ulmo: Voronwë.
In older versions of The Silmarillion, Ossë was a Vala in his own right, and often opposed the will of Ulmo.