For some reason it seems that an unusual number of people (i.e., more than one) on the ruby-talk mailing list have referred to Ruby as a functional language, as if that were is primary characteristic (as opposed to the far more common view of Ruby as quintessentially object-oriented).
Lisp has been object-oriented since its beginnings--when there were no user-defined data types at all, and when there were consequently no distinctions between internal and external interfaces (other than those that programmers contrived for their own use).
I think "object-oriented" is a metaphor, philosophy, or methodology that guides how you think about programs. The original source of the metaphor seems rooted in the concept of object identity. Even the earliest Lisps exhibit this concept in at least two powerful ways: First, symbols are interned, so when you name a symbol you are not just asking for any old object of type symbol with the indicated name, but rather for some very specific symbol. Second, some objects that are not interned can still be modified by side-effect, just as real-world objects can be, leading to important phenomena such as structural sharing (and even circular reference) and informational locality.
To me, the essence of object-oriented programming is captured by the idea that objects are things with an identity that extends uniformly throughout a program. The programming system is not free to reclaim my object and substitute a structurally similar but detectably different object on the mere hope that it will be equally satisfactory. There is a contract between the programming system and me about what the salient aspects of my objects are, and they are preserved throughout those objects' observable lifetime within my program.
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