In the Nahal Hever Greek manuscript of the Minor Prophets (8HevXIIgr) found among the DSS, the manuscript doesn't use the Nomina Sacra for Θεος but instead has it written out in full (Θεος, Θεω, Θεον etc.), and where we'd usually see the Nomina Sacra for Κυριος, this DSS Greek manuscript uses the Tetragrammaton in Paleo-Hebrew letters.
The rest of the Greek manuscripts found among the DSS are too fragmentary to determine what was used in them (4Q119 (4QLXXLeva), 4Q121 (4QLXXNum), 4Q122 (4QLXXDeut) - although the reconstruction of 4Q119/4QLXXLeva on Logos shows that there was definitely space for Θεος to be written out in full), bar one final one of Leviticus - 4Q120/pap4QLXXLevb. Whilst it's too fragmentary to see how it used Θεος, we can quite easily see that rather than what we'd usually expect for the Tetragrammaton (either Paleo-Hebrew letters or the Nomina Sacra), this manuscript has it transliterated as Ιαω.
So there really isn't a single manuscript in existence (that I know of) that omits any letters from within Θεος or Κυριος that corresponds with the idea of omitting the 'o' from 'God', for as Mr. Lim has pointed out, 'God' is Elohim in the Tanakh, and isn't used as a proper name, although God is (wrongly) used as a proper name in English by quite a few people.
The Nomina Sacra are probably one of the most universal, yet completely uncommented upon Christian scribal habits in History, for we really don't know all that much about them; when they first appeared; who started the trend off; whether it really was just a Christian scribal habit or whether they adopted it extremely quickly. But I don't think the Nomina Sacra correspond at all to the current trend of writing 'God' as 'G-d'. The most universal ones that appear in all Christian Greek manuscripts (those for Χριστος, Θεος, Κυριος, Ιησους) are all certainly considered 'Divine' names/titles, which would account for their special status as being singled out in the text.
Plus, whilst there did end up being 15 main used Nomen Sacrum (as Mr. Penner has pointed out), their use in pre-fourth century Greek manuscripts are quite sporadic, with several manuscripts using the Nomina Sacra for one word, and then in the next sentence not using the Nomina Sacra for the other instances of the word. I'd see the list of Nomina Sacra in pre-fourth Century Greek manuscripts of the NT over on Wikipedia - http://en.wikipedia.org/wiki/Nomina_sacra
It should be noted that the Nomen Sacrum for Ιησους also appears in Septuagint Greek Manuscripts, as the following link to ms2648 shows (it looks like IHC with an overline): http://www.schoyencollection.com/GreekN ... ms2648.jpg