[cygx]

Imo declarative, functional and OO are somewhat but not really orthogonal concepts.

A mathematical (ie pure) function is declarative in nature as it defines a relation between sets. However, in practice you often use it imperatively, ie as a means to get an output from some input.

An object is imperative in nature as it encapsulates mutable state. However, the set of messages defines an abstract interface which is arguably declarative.

In principle, a purely functional language cannot be object-oriented, and a purely object-oriented language cannot be functional. In practice, this doesn't matter as pure languages are rare.