Ossë (from the Valarin Ošošai, Oššai) was a Maia associated with Ulmo.
A spirit of the sea in the service of Ulmo, he guarded the waters around Middle-earth. Uinen was his wife, and he was a friend of Círdan the Shipwright. During the Years of the Trees, he briefly entered into the service of Melkor and began causing wanton storms which made travelling by sea particularly unsafe. He was persuaded to stop by Uinen, but his taste for storms didn't quite disappear. When Ossë grew restless in the Second Age, the mariners of Númenor would call for help from Uinen to quiet his storms.
Ossë was a friend of the Sindar, and was valued as high as the Valar by them.
In older versions of The Silmarillion, Ossë was a Vala in his own right, and often opposed the will of Ulmo.