General antonyms:
* Plain
* Simple
* Basic
* Unpretentious
* Spartan
* Unadorned
* Bare
More specific antonyms:
* Functional (if referring to the purpose of the object)
* Minimalist (if referring to a style of design)
* Rustic (if referring to a natural, unpolished style)
The best antonym will depend on the specific context in which you are using the word "ornate."