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/.