Specifically, what's the consonant symbol equivalent for the glide/semivowel /o̯/, like how /i̯/ is equivalent to /j/?
There is no simple IPA letter for [o̯] (or /o̯/). You can see the complete chart of IPA symbols on the following website: http://www.internationalphoneticalphabet.org/ipa-charts/ipa-symbols-chart-complete/
The IPA obviously only has a limited number of letters that don't cover all possible human speech sounds. In phonetic transcriptions people just write [o̯]; in a phonological transcription—well, non-syllabic consonantal [o̯] is not a common sound [by which I mean, I don't know of any language that is uncontroversially analyzed with a phoneme /o̯/—see user6726's comment), so it might be analyzed in various ways depending on its distribution and the rest of the inventory: as /w/, or an allophone of /l/, or something like that. (In a narrow transcription, a way of indicating that the phoneme /w/ is pronounced as the phone [o̯] would be to use the "lowered" diacritic, [w̞].)
Height contrasts between semivowels are much less common than height contrasts between vowels. In fact, while [i̯] and [j] are as you say pretty much equivalent, in many languages the /j/ phoneme may in fact be a bit higher on average than the /i/ phoneme—this is one reason why it's possible in some languages to have a sequence like /ji/ that is distinct from /i/ or /iː/.
2FYI, the reason is mildly complicated. First, substantial political and bureaucratic effort is required to add any new symbol. Second, a desideratum of adding a symbol is that the sound must be clearly phonemic in some language (and there is variation what scholars think "phonemic" means). It's not just uncommon, it is not clearly phonemic.– user6726Mar 2, 2017 at 17:45