The /ɒ/ vowel is usually spelled <o> but we also find <a> after <w> in Wally and wad. The /ʊ/ vowel is spelled <oo>, <u>, or <oul> in could.
This contrast is between two short vowels, both rounded and fairly close in the mouth. However, it is not a problem for any particular set of learners, perhaps because most languages have a distinct /o/ and /u/ set of phonemes to which these sounds are allotted.
The only taboo word pair is cock/cook.
Interesting pairs include:
The pair posh/push became familiar in 2005 in a slogan arising from media reports about the rise in the number of Caesarian births. Middle class women, it was said, were choosing Caesarians because they were "too posh to push".
The density value is 3.67%. The pairs make 37 semantic contrasts, giving a loading of 51.4%.
bollock bullock bollocks bullocks botch butch botcher butcher botchers butchers box books cock cook cocked cooked cocking cooking cocks cooks cocker cooker cockers cookers cocky cookie cod could crock crook crocked crooked crocking crooking crocks crooks folly fully god good gods goods godly goodly godlier goodlier godliest goodliest hock hook hocked hooked hocking hooking hocks hooks hockey hooky hod hood hods hoods knock nook knocks nooks lock look locked looked locking looking locks looks locker looker lockers lookers lockout lookout lockouts lookouts pod pud pods puds podding pudding poll pull polls pulls poly pulley polys pulleys posh push posher pusher posse pussy pot put pots puts rock rook rocked rooked rocking rooking rocks rooks rockery rookery rockeries rookeries rocky rookie shock shook shod should sot soot sots soots stop stoep stops stoeps wad wood wads woods wadded wooded wadi woody Wally woolly workbox workbooks
John Higgins, Shaftesbury, November 2009