2 letters:
or

3 letters:
cor, mor, nor, oar, ore, orr, por, tor, war

4 letters:
abor, boar, boer, bohr, bore, core, corr, dior, door, dorr, flor, fore, four, gore, hoar, igor, lohr, lore, more, pore, pour, roar, saur, soar, sore, thor, tore, torr, wore, yore, your

5 letters:
abhor, adore, afore, angor, chlor, chore, corps, crore, decor, floor, fomor, frore, gabor, moore, score, senor, shore, smore, snore, spore, store, swore, timor, whore

6 letters:
and/or, ardour, ashore, bangor, before, bezoar, chador, drawer, encore, galore, ignore, indoor, inocor, lahore, lenore, mentor, mysore, prewar, salvor, schorr, uproar, vapour

7 letters:
anymore, bedsore, candour, centaur, deplore, ecuador, explore, eyesore, humidor, implore, inshore, languor, matador, mevacor, mirador, moidore, onshore, outdoor, outpour, outroar, picador, pledgor, postwar, rapport, restore, tandoor, tussore

8 letters:
albacore, allosaur, backdoor, barndoor, barosaur, claymore, dinosaur, evermore, ex-mayor, folklore, footsore, forswore, hardcore, labrador, minotaur, offshore, outscore, phosphor, pinafore, polypore, salvador, seashore, sycamore, theodore, therefor, trapdoor, twoscore, unprofor, wherefor, zoospore

9 letters:
apatosaur, archosaur, baltimore, bangalore, barrymore, bookstore, bucentaur, cared-for, carnivore, carnosaur, commodore, comprador, drugstore, embraceor, foreshore, fourscore, frugivore, guarantor, hackamore, hadrosaur, hard-core, herbivore, hoped-for, lakeshore, livermore, longshore, madrepore, manticore, nevermore, nothosaur, pterosaur, singapore, soft-core, sophomore, splendour, stegosaur, stevedore, therefore, thermidor, warrantor, wherefore

10 letters:
alongshore, ankylosaur, apishamore, battledore, blastopore, brontosaur, ceratosaur, heretofore, ictodosaur, longed-for, looked-for, man-of-war, megalosaur, pelycosaur, pisanosaur, plesiosaur, re-explore, seismosaur, smoothbore, superstore, threescore, titanosaur, troubadour, tug-of-war, underscore, wished-for

11 letters:
closed-door, corythosaur, dromaeosaur, forevermore, furthermore, herrerasaur, ichthyosaur, kinetochore, out-of-door, styracosaur, theretofore, twenty-four, two-by-four, tyrannosaur, uncared-for, unhoped-for, yearned-for

12 letters:
conquistador, door-to-door, hereinbefore, psittacosaur, staurikosaur, trichobezoar, uncalled-for, unlooked-for, unwished-for

13 letters:
argentinosaur

14 letters:
circumferentor, coach-and-four

16 letters:
pachycephalosaur

1 letter:
r

2 letters:
ar, er, r.

3 letters:
bur, mir, jar, npr, air, par, fur, mar, err, far, gar, ear, car, ayr, sir, are, ire, cur, bar, tar, per, fir

4 letters:
deer, bear, pare, sour, slur, dare, poor, lour, rear, wear, weir, fair, year, tyre, amir, dire, veer, were, spar, gear, hire, hare, lear, hour, star, lair, boor, near, tier, sure, mare, jeer, pair, moor, hair, sear, meir, cure, fare, herr, doer, fear, tear, carr, scar, bier, parr, care, burr, fire, czar, wire, rare, dour, bare, tsar, blur, birr, lyre, hear, sere, ware, mire, beer, char, spur, emir, dear, pure, ruhr, seer, lure, mere, purr, heir, pear, peer, stir, here, tour, barr, adar, pier, fahr, fehr, gere, jour, voir

5 letters:
khmer, loire, aware, blair, assur, barre, churr, queer, skier, glare, blare, ameer, sitar, drear, ne'er, qatar, whirr, flair, scour, defer, cheer, flare, afire, amour, dower, demur, steer, safar, share, shirr, babar, stair, unbar, we're, there, snare, chirr, stare, inure, myrrh, spear, their, sneer, inter, quire, shear, swear, spare, dakar, incur, shire, sheer, dinar, bazar, chair, flour, zaire, spoor, occur, refer, scare, blear, starr, clear, moire, infer, deter, spire, smear, murre, recur, where, cigar, asarh, ashir, astir, athar, bihar, damar, debar, freyr, greer, grier, kotar, lamar, maher, sapir, selar, sherr, vanir, vidar, yasir

6 letters:
lekvar, appear, unfair, impure, secure, velour, impair, insure, guitar, career, confer, sphere, midair, ensure, affair, assure, square, immure, bazaar, aspire, adjure, pierre, endear, disbar, unsure, endure, squire, retire, beware, concur, rehear, repair, prayer, cohere, severe, claire, detour, mature, qintar, poseur, prefer, manure, you're, veneer, demure, revere, adhere, savour, voyeur, allure, admire, expire, abjure, aflare, anshar, bonior, cottar, devoir, eclair, exmoor, fakeer, inhere, isobar, jamjar, kaffir, lontar, menhir, nuphar, o'hare, pindar, rigour, safire, sambar, sambur, secpar, shamir, shmear, vigour

7 letters:
austere, compere, verdure, myanmar, respire, pasteur, compare, unclear, liqueur, premier, prepare, schmear, require, pioneer, they're, inquire, saviour, masseur, cashier, inspire, sincere, bizarre, couture, ensnare, declare, compeer, danseur, succour, obscure, unaware, fervour, procure, flavour, affaire, despair, astaire, besmear, bonaire, bonheur, bugbear, co-star, cooccur, flexure, forbear, gravure, guinier, hachure, houhere, kildeer, minibar, moliere, nervure, rancour, samovar, shophar, soilure, tangier, tarsier, tonsure, upstair, vitharr, wafture

8 letters:
brochure, grimoire, overhear, nightjar, reappear, brumaire, engineer, reinsure, seigneur, souvenir, cavalier, saboteur, mutineer, frontier, frotteur, grandeur, immature, domineer, frimaire, voltaire, monsieur, reassure, debonair, forswear, premiere, perspire, transfer, abampere, abattoir, amanpour, baccilar, coiffeur, coiffure, coinsure, derriere, elaphure, encolure, footgear, footsure, handwear, hot-wire, killdeer, lavalier, longueur, markhoor, microbar, muleteer, open-air, overbear, paramour, parterre, peignoir, scimitar, sinecure, t-square, tamanoir, testiere, voussoir, wheatear, wirehair

9 letters:
balladeer, buccaneer, puppeteer, superstar, chevalier, raconteur, disappear, solitaire, insincere, musketeer, persevere, premature, gondolier, volunteer, profiteer, disrepair, interfere, bandolier, racketeer, sightseer, chauffeur, financier, brigadier, belvedere, bandoleer, basketeer, cannoneer, chinaware, debonaire, downstair, gadgeteer, gazetteer, guinevere, hekmatyar, horsehair, kala-azar, megathere, pistoleer, presbyter, privateer, pushchair, rest-cure, sloganeer, sonneteer, trochlear, wasteweir

10 letters:
quadrature, bombardier, chandelier, charioteer, escritoire, commandeer, foursquare, auctioneer, air-to-air, baudelaire, belshazzar, blackamoor, carabineer, cockleburr, debonnaire, embouchure, gondoliere, halberdier, harpooneer, kieselguhr, scrutineer, secretaire, uintathere

11 letters:
pamphleteer, billionaire, litterateur, doctrinaire, millionaire, connoisseur, rectilinear, interlinear, candidature, dyaus-pitar, gomphothere, liquidambar, milliampere, rotogravure, supersedure, ulaanbaatar, volt-ampere

12 letters:
entrepreneur, restaurateur, aide-memoire, heliogravure, photogravure, pied-a-terre, savoir-faire

13 letters:
questionnaire, conventioneer, conservatoire, laissez-faire, porte-cochere

14 letters:
concessionaire, commissionaire, surface-to-air

15 letters:
kilovolt-ampere