Two patterns of alternation affect the phonemes /a/ and /o/ in Subanon, an underdocumented Austronesian language spoken in the southern Philippines. Under suffixation, /a/ becomes /o/ in the antepenult (Pattern 1), and /o/ becomes /a/ in the penult preceding a palatal glide (Pattern 2). Pattern 1 has no apparent synchronic motivation, but comparative evidence shows that Proto-Subanen *a weakened to schwa when placed in pretonic position through suffixation, and that schwa from any source then became Subanon /o/. Pattern 2 is similar to a Subanon process called "partial vowel harmony assimilation," as well as to the alternation of final -əy/-əw with penultimate -ay/-aw in Western Bukidnon Manobo. However, in both cases, these processes turn out to be unrelated. In conclusion, Pattern 2 shows no clear synchronic and diachronic motivation, and we are left with a descriptive statement without an explanation.