Indeed they can.  But I think what you really want is a PI containing the OID to be placed *before* the element.  Doing it that way means that you will have the OID available before you even begin to parse the start-tag.  That seems to me to be better than having it as the first attribute.

Fortunately, that can always be done: you can have a PI anywhere except inside another PI, a tag, an entity reference, a CDATA section, or a comment.