Is the 3rd person singular of the "passato remoto" tense of "portare" (lui portò) pronounced [por'tɔ] ? It should be, but I (maybe incorrectly) hear [por'to] in Google Translate (I used the text "lui portò. io porto e porterò" to easily compare the [o] and [ɔ] sounds in the different conjugations). There is no IPA pronunciation or audio available in the corresponding Wiktionary page. I also checked http://www.dizionario.rai.it/static.aspx?treeID=37&pg=7 , but it has a small cross right next to the Ò sound and I have no idea what that means.
The accented “ò”s in Italian pose no doubts. The grave accent (`) always, with no exception, denotes an open vowel /ɔ/. The same holds for “è” (/ɛ/) versus “é” (/e/). You might write “ó” (for /o/), but it's only used to explicitly clarify the pronunciation of some word (bótte as meaning “barrel”, say) and not in actual spelling. Unstressed “o”s and “e”s are always closed. So there is no ambiguity here.
(The above, as always, holds for Standard Italian. Usual disclaimer about the wide variability in different regional Italians.)
As for DOP's (and other dictionaries') notation: what matters is here the grave accent, which in itself denotes an open wovel. The small “+” just means that this word ending generates a raddoppiamento sintattico in the word that follows.