What's this "O<" (capital O and subscript less-than sign) Sinological IPA transcription?
I think that [ʊ] is associated with "latter part" of the vowel (diphthong) trascribed as -ou or -iu in pinyin.


  What does [ʊ] have to do with O


[ʊ] is usually used to represent a sound like the vowel in "good" or "hood"
 (American English). This sound is also used to describe the second part of the diphthong in words like "so" and "bow", something like [soʊ] and [boʊ], respectively.

O&lt;5 and iO&lt;5 correspond to -ou and -i(o)u in pinyin, I think. By this sort of analysis, as the [ʊ] comes together with an [o] in like in the English example above, it looks like it got abbreviated with a weird symbol.

While I think that this is probably just be a typographical error as pointed out in the comments, it is important to remember that even a phonetic transcription system like IPA is a practical tool, not an airtight system for describing speech sounds. Relatively narrow IPA transcriptions might describe two languages as having an [a] vowel, for example, but the actual sounds realized in these languages (or even saying the same word in different contexts) necessarily will differ, as sound is not digital. What is essential with any transcripiton system is to capture meaningful contrasts, which pinyin seems to do an adequate job at.