The /ʌ/ vowel is spelled with <u> throughout. The /ɔɪ/ vowel is spelled <oi> or <oy>.
This is a contrast between a short vowel and a diphthong with fairly close starting positions but distinguished by lip-rounding. It does not seem to be a problem for many learners.
Interesting pairs include:
cluster cloister
cunning coining
The density figure is 3.82%. The set makes 15 semantic contrasts giving a loading of 50%.
bud buoyed buzz boys buzzbuoys cluster cloister clustered cloistered clustering cloistering clusters cloisters cuff coif cuffs coifs cull coil culled coiled culling coiling culls coils cunning coining cut quoit cuts quoits fussed foist gutter goitre gutters goitres just joist mull moil mulled moiled mulls moils mussed moist punt point punted pointed punting pointing punts points punter pointer punters pointers
John Higgins, Shaftesbury, December 2009