Osse -Psycho of the Seven Seas

[Legate of Amon Lanc]

Don't forget the fact that Ossë was once persuaded by Melkor, and as it is said, he somewhat "remained tainted":

Originally Posted by Valaquenta

It is said that in the making of Arda he endeavoured to draw Ossë to his allegiance, promising to him all the realm and power of Ulmo, if he would serve him. So it was that long ago there arose great tumults in the sea that wrought ruin to the lands. But Uinen, at the prayer of Aulë, restrained Ossë and brought him before Ulmo; and he was pardoned and returned to his allegiance, to which he has remained faithful. For the most part; for the delight in violence has never wholly departed from him, and at times he will rage in his wilfulness without any command from Ulmo his lord.

The part about doom of Mandos, though, as it was quoted here, brings another aspect into play: sometimes, Ossë could do things that Valar (or Mandos) wish, though his master Ulmo, the "secret voice that gainsayeth", seems not to want this (though it is clear from his speech to Tuor that it only seems that he goes against the will of Valar).