The /ɑ/ vowel is usually spelled <ar> but we also find <a> in ta and <aa> in baa. The /ɔɪ/ diphthong is spelled <oi> or <oy>.
This contrast is between a long vowel and a diphthong. The starting tongue positions are fairly close but confusions are unlikely.
Interesting pairs include:
carnage coinage
harden hoyden
The density value is 6.12%. The pairs make 26 semantic contrasts, giving a loading of 54.2%.
baa boy baas boys baa buoy baas buoys baa'd buoyed baaing buoying bar boy bars boys bar buoy barred buoyed bars buoys calf coif Cali coyly Carl coil carnage coinage cart quoit carts quoits fa Fowey fast foist fasted foisted fasting foisting fasts foists garter goitre garters goitres harden hoyden hardens hoydens jar joy jarred joyed jars joys khan coign khans coigns lards Lloyd's larn loin larns loins marl moil mast moist parrs poise pars poise parse poise parsed poised parses poises parsing poising rah Roy ta toy tar toy tarred toyed tars toys vast voiced
John Higgins, Shaftesbury, November 2009