There is some variation in ἄλφα /Ἄ, so it may not be significant. Look at
Alan Bunnig's site and dial in your verse.
I think that having the article in front of it - regardless of whether it is spelt out or not - makes it clear enough that it's a letter. If the article were masculine "ὁ" then it may have been read as πρῶτος "first". The symbolic significance of ὦ would be ἔσχατος "last (in a sequence)", or τέλος "last (without anything following)".
LSJ ὦ wrote:The name of the letter was τὸ ὦ (perispom. acc. to Hellad. ap. Phot.p.530B.), cf. Achae.33.3, Pl.Phdr.244d, Cra. 420b, Tht.203c: after the loss of the distinction betw. long and short vowels, ο and ω had the same pronunciation; they begin to be confused in Papyri of iii B. C. (“οἰκωνόμου” PRev.Laws 50.22 (iii B. C.)), but the name ω μέγα appears first in later Greek, Theognost.Can. 13; “κατὰ σχῆμα διπλοῦ ω_ ἤτοι μεγάλου” Eust.869.26; “οἱ δὲ περὶ Ἀρίσταρχον αὐτὸ τὸ ποτήριον ω_ μέγα εἶναί φασιν, ὁποῖν ἴσως τὸ κατὰ δύο υ_ ἐσχηματισμένον” Id.869.29; ἐν τῷ ω_ μεγάλῳ under omega (in a lexicon), Id.1828.49: “διὰ του^ ω_ μεγάλου” Hdn.Epim.208.