Phonetics and the IPA 5, Exercise 3: monophthong or diphthong

hit   monophthong [ɪ]. The quality stays the same throughout (Wikipedia's
      own example).
boy   diphthong [ɔɪ]. The quality glides from one vowel to another (also
      Wikipedia's example).
oh    diphthong in the source's transcription /oʊ/, a glide from [o] towards
      [ʊ]. Caveat: in some accents the same word is a steadier vowel; this
      course treats accent differences in Chapter 7.

Evidence: say the vowel slowly and hold it. If your tongue and lips stay
still, it is a monophthong; if they move to a new position, it is a
diphthong. The source adds that the diphthong/triphthong choice depends on
whether you analyse the sequence as one phoneme or several.