There are three vowels that might be listed as diphthongs in Brunei Malay: /aj/, /aw/ and /uj/ (Mataim 2007: 36). The following words illustrate their occurrence:
These three diphthongs usually occur in word-final position, though kaola, which is listed in DBP (2007: 151) as the first person pronoun when talking to people of noble rank, has /aw/ in its initial syllable.
A diphthong can never be followed by a consonant within the same syllable (so kaola is /kaw.la/), and the basic syllable structure of Malay is CVC, so the diphthongs are regarded as a monophthong vowel followed by a glide. This is consistent with most analyses of Malay (e.g. Clynes 1997, Clynes & Deterding 2011), and Asmah (1983: 44) similarly notes that although diphthongs are quite common in Malay, phonemically they can be analysed as a monophthong vowel followed by a glide.
Two issues remain with these vowels: Why is /uj/ spelled as ‘oi’? And is there a distinction between /oj/ and /uj/?