The /ɔɪ/ diphthong is spelled <oi> and <oy>. The /aʊ/ diphthong is spelled <ow> or <ou>.
This is a contrast between two diphthongs with different starting positions and different directions of movement. It is not a problem for learners.
The density figure is 4.97%. The set makes 20 semantic contrasts giving a loading of 51.3%.
alloy allow alloyed allowed alloying allowing alloys allows avoid avowed boy bough boys boughs boy bow boys bows buoy bough buoy boughs buoy bow buoyed bowed buoying bowing buoys bows cloyed cloud coil cowl coiling cowling coils cowls coiled cowled coir cower coy cow foil foul foiled fouled foiling fouling foils fouls groined ground joist joust joists jousts moist moused oil owl oils owls ploy plough ploys ploughs Roy row royal rowel royals rowels soy sow void vowed
John Higgins, Shaftesbury, December 2009