The /ɑ/ vowel is usually spelled <ar> but we also find <a> in khaki and pasha. The /ʊ/ vowel is spelled <oo> or <u>, and <ou> in should.
This contrast is between a long and a short vowel, both back vowels but fairly well separated in the mouth. It is unlikely to cause many problems for learners.
There is one set of pairs involving a taboo word: fart/foot.
Interesting pairs include:
The density value is 1.99%. The pairs make 19 semantic contrasts, giving a loading of 48.7%.
bark book barked booked barking booking barks books barley bully card could fart foot farted footed farting footing farts foots guard good guards goods hard hood hark hook harked hooked harking hooking harks hooks khaki cookie lakh look lakhs looks lark look larked looked larking looking larks looks nark nook narks nooks parch putsch parches putsches pardon pudden pardons puddens parley pulley parleys pulleys part put parts puts pasha pusher pashas pushers shard should shark shook starred stood
John Higgins, Shaftesbury, November 2009