这两个动词均含“装饰”之意。

decorate : 普通用词，指对人或物进行装饰，使之更加完美。

They decorated the wedding car with ribbons and flowers.

他们用彩带和鲜花装点婚车。

They decorated the room with flowers and balloons.

他们用花和气球装饰了房间。

The cake was decorated to look like a car.

这蛋糕装饰得像一辆汽车。

ornament : 指装饰以精美之物，使某处或某物增添美丽的或景色。

She ornamented her letters with little drawings in the margin.

她在信的页边空白处画上小图案来作装饰。

a room richly ornamented with carving

雕饰得富丽堂皇的屋子