As far as I've read, bibliography items are typically inserted into
the bibliography as you need them. Therefore, one may assume that they
are interested in the fashion you want them displayed - in the order
they appear in the text. BibTeX and natbib alike then use different
ways of sorting the bibliography contents to specific requirements.
Consequently, if you leave the list unsorted (using the unsrt or
unsrtnat style), it should appear in your required order. However, I
doubt whether there is a 'sorting' mechanism for sorting according to
your citation sequence. I say 'doubt' because honestly I have no
idea...
Werner