Sure it is.  It's an additional convention that a specialized XML processor (and serializer) can take advantage of if it understands it.

If I understand the oid attribute correctly, the receiver may maintain a mapping from oids to element nodes in its internal representation, and what the sender must guarantee is that if the oid is the same, the other attributes, their values, and the content of the element are the same as before.  At least, that is the way that ETags work.  Is that correct?