No.
I should stop there, but I'll at least go on to ask if you mean
"unequivocally synonymous in the Aristotelian sense?
Does this question relect any real familiarity with Greek? All of these
words can bear a variety of meanings and they can overlap to some extent.
If you really want a good differentiation of them as philosophical terms,
you could do worse that consult F.E.Peters' _Dictionary of Greek
Philosophy_ (or some such title): it's very good on tracing the historical
development of such terms as these.
PHUSIS (we generally transliterate as FUSIS here) has two most common
senses, both deriving from a botanical metaphorical use of the word meaning
"plant growth." It is used in ancient thought for (1) the whole world-order
conceived as a living organism, like "nature" in one of its major senses;
(2) the constitutent elements and inherent capacities of any natural thing
determined by the factors underlying its growth to be what it is. There are
other senses also.
ONTOS is simply the genitive singular participle of the Greek verb EIMI,
meaning "be" From the participial stem ONT- are derived all the words in
English having to do with the nature of reality, such as, most simply,
"ontology."
OUSIA: you could do worse than consult Aristotle's Metaphysics 4 or 5
(whichever is his dictionary of metaphysical terms). The two chief
Aristotelian uses are for (1) "substance"--a real thing intelligible in
terms of underlying material and distinct form, and (2) the "reality" of
any substance in the first sense, which is to say, its intelligible form,
or MORFH.
Carl W. Conrad
Department of Classics, Washington University
One Brookings Drive, St. Louis, MO, USA 63130
(314) 935-4018
cwconrad@artsci.wustl.edu OR cwc@oui.com
WWW: http://www.artsci.wustl.edu/~cwconrad/