The o in ego is short as well (so far, I haven't found a single instance in poetry where it's long, so I guess it pretty much reflects actual speech) ... that's what I mean by iambic shortening happening so often.
Oh! That's interesting... I looked up the entry in L&S and it does mention "ŏ always in poets of the best age, as Cat., Verg., Hor., etc.; ō ante-class. and post-Aug., as Juv. 17, 357; Aus. Epigr. 54, 6". Most Latin textbooks, at least those in English, use the -ō variant. I was able to find an instance of it in Martial IV.72, but Martial otherwise mostly uses short -ŏ:
— u u | — — | — u u | — u u | — u u | — —
‘Aes dabŏ prō nūgīs et emam tua carmina sānus?
— — | — u u | — | — u u | — u u | —
Nōn’ inquis ‘faciam tam fatuē.’ Nec egō.
(EDIT: the last line is actually "(u) | x" for egō/egŏ, as corrected by Pacifica and Bitmap below.)
And yet again here I note the short -ŏ of dăbŏ...