The /ɒ/ vowel is spelled <o>, or <a> after <w> in was and want. The /əʊ/ diphthong is spelled <oCe>, <oa>, <ow>, <ew> in shewn and <au> in the loan word gauche, and occurs in the letter name O. Notice the partial homographs job/Job and polish/Polish.
This contrast is between a short rounded vowel and an unrounded diphthong. The starting position of the tongue in the mouth is fairly close. Many languages (and indeed many accents of English other than RP) have no diphthong corresponding to /əʊ/ and will substitute a monophthong [o] which will be heard as /ɒ/ or /ɔ/, and the spelling is likely to encourage this. All this makes this a recognised problem for speakers of Spanish, Italian, Greek, Arabic, Chinese and Swahili, and probably for many other learners too.
One mildly taboo word pair is cock/coke.
Interesting pairs include:
bonny bony
comma coma
dotty dhoti
honour owner
lolly lowly
robot rowboat
The density value is 3.6%. The pairs make 113 semantic contrasts, giving a loading of 48.9%.
bloc bloke blocs blokes block bloke blocks blokes blot bloat blotting bloating blotted bloated blots bloats blotter bloater blotters bloaters bod boed bod bowed bodied boded boggy bogey bond boned Bonn bone bonny bony bonnier bonier bonniest boniest bossed boast choc choke chocs chokes chock choke chocked choked chocking choking chocks chokes choler cola cholers colas collar cola collars colas clock cloak clocked cloaked clocking cloaking clocks cloaks cock coke cocked coked cocking coking cocks cokes cocks coax cod code codded coded codding coding cods codes col coal cols coals comma coma commas comas comma comber commas combers con Colne con cone conned coned conning coning cons cones Connie coney consols consoles cop cope copping coping copped coped cops copes Copt coped cost coast costed coasted costing coasting costs coasts cot coat cots coats cox coax coxed coaxed coxes coaxes coxing coaxing crock croak crocked croaked crocking croaking crocks croaks dodge doge dodges doges doll dole dolled doled dolling doling dolls doles doss dose dossed dosed dosses doses dossing dosing dot dote dots dotes dotted doted dotting doting dotty dhoti foggy fogey fond phoned fox folks foxy folksy god goad gods goads golly goalie gosh gauche got goat grot groat grots groats hocks hoax hod hoed holly holy Hon hone honour owner honours owners hop hope hopped hoped hopping hoping hops hopes job Job jobs Jobs John Joan Johnny Joanie knobble noble knobbles nobles knobbly nobly knot note knotted noted knotting noting knots notes lob lobe lobbed lobed lobs lobes Lodz loads loft loafed lolly lowly lop lope lopped loped lopping loping lops lopes mock moke mocks mokes Mod mode Mods modes model modal models modals moll mole molls moles mop mope mopped moped mopping moping mops mopes nod node nods nodes odd ode odds odes odd owed odder odour off oaf on own ox oaks oz O's pock poke pocked poked pocks pokes polish Polish poll pole polls poles pop Pope pops Popes rhomb Rome rhomb roam rhombs roams rob robe robbed robed robbing robing robs robes robot rowboat robots rowboats rod road rods roads rod rode rod rowed Ron roan roster roaster rosters roasters rot rote rot wrote rotter rotor rotters rotas shod shewed shone shewn slop slope slopped sloped slopping sloping slops slopes smock smoke smocked smoked smocking smoking smocks smokes soccer soaker sock soak socked soaked socking soaking socks soaks sod sowed Sol Seoul sop soap sops soaps sopped soaped sopping soaping soppy soapy soppier soapier soppiest soapiest stock stoke stocked stoked stocking stoking stocks stokes toddy toady toddies toadies Tom tome top tope topped toped topping toping tops topes topper toper toppers topers tossed toast tot tote tots totes totted toted totting toting unwanted unwonted vol vole vols voles wad woad want won't wanted wonted was woes