2 letters:
og

3 letters:
bog, cog, dog, fog, jog, log, ogg, pog, rog

4 letters:
acog, agog, blog, clog, flog, fogg, frog, grog, haag, haug, hogg, krog, maag, mogg, plog, skog, slog, smog, waag, zogg

5 letters:
-agog, barog, bayog, befog, bihag, blogg, bulog, cosag, craag, dague, defog, delag, dogge, drcog, embog, eqlog, facog, frogg, fseog, gen.g, glogg, graag, grogg, gulag, hilog, hogge, lague, locog, logge, magog, morag, parag, pdogg, phlog, pirog, pogge, progg, recog, rogge, scrog, silog, skrog, smaug, socog, splog, sprog, strog, tague, tofog, wolog, zaugg, zilog

6 letters:
-logue, agogue, analog, anurag, applog, augcog, balrog, bandog, bigdog, buldog, catdog, clague, condog, coydog, croagh, dcslog, dildog, dos/4g, eggnog, epilog, fogdog, frogge, funlog, gaplog, gundog, hamdog, hotdog, ibague, lapdog, medlog, moblog, objlog, parlog, photog, pi-dog, poplog, prague, precog, prelog, prolog, putlog, pyedog, quahog, quohog, ratdog, sawlog, seadog, seahog, sundog, svarog, syslog, tablog, taclog, tetaug, unclog, weblog

7 letters:
-agogue, antifog, antilog, backlog, bairaag, beamdog, belobog, bibliog, bonedog, brewdog, bulldog, calinog, camfrog, caradog, coondog, corndog, datadog, dazhbog, dekalog, demagog, dipolog, eclogue, egg-nog, firedog, graylog, gun dog, hangdog, hertzog, hirulog, horndog, hot-dog, icefrog, insarag, kenalog, lexicog, lifelog, log-log, longdog, menulog, milblog, monolog, moondog, nidhogg, nig-nog, oceanog, overdog, pedagog, pie-dog, pug-dog, pye-dog, quahaug, roadhog, rsyslog, sandhog, schelog, sea dog, semilog, sherdog, sinulog, spurdog, starlog, synagog, tagalog, templog, theolog, top dog, toy dog, vandaag, verilog, warblog, wolfdog

8 letters:
abominog, analogue, angus og, anti-fog, antismog, armikrog, ballarag, bird-dog, bird dog, bullfrog, calbayog, cant dog, chemofog, chilidog, coachdog, den haag, dialogue, duologue, epilogue, fire-dog, footslog, freakdog, hang-dog, hedgehog, hop-frog, housedog, isologue, leapfrog, liveblog, madrague, microlog, ortholog, peat bog, pettifog, pflp sog, portague, q-analog, sheepdog, shockdog, sled dog, spacehog, swisslog, synagoge, sys-frog, toadfrog, travelog, treefrog, tremadog, underdog, watchdog, waterdog, waterlog, wild dog, wolf-dog, woodfrog, yon-rogg, yule log

9 letters:
bull-frog, bulldogge, catalogue, changelog, cheeselog, chernabog, chili dog, coach dog, cs-prolog, decalogue, demagogue, devil dog, devprayag, flyingdog, glottolog, golliwogg, great dog, groundhog, guard dog, guide dog, hemagogue, holagogue, homologue, hound dog, hyena dog, ic-prolog, idealogue, leap-frog, lm-prolog, magalogue, melologue, menagogue, metagogue, microblog, monologue, mummachog, mummichog, nate dogg, nu-prolog, pedagogue, photoblog, qu-prolog, sb-prolog, screw log, sheep dog, shot-clog, sinologue, steyr aug, superdawg, synagogue, tree-frog, tree frog, trialogue, true frog, videoblog, water dog, whole-hog, wood-frog, zytglogge

10 letters:
astrologue, attack dog, badger dog, bandar-log, cholagogue, copenhague, emenagogue, eskimo dog, ethnologue, euchologue, grass frog, green frog, haemagogue, horny frog, hydragogue, hypnagogue, lactagogue, lithagogue, little dog, monkey dog, myriologue, mystagogue, ninkharsag, objvprolog, pariah dog, patent log, philologue, physagogue, pnu-prolog, police dog, poodle dog, porthmadog, sheep frog, sialagogue, sledge dog, steven ogg, swi-prolog, travel-log, travelogue, yellow-dog

11 letters:
brycheiniog, cardiologue, cat-and-dog, chorus frog, dog-eat-dog, ecrc-prolog, electrosmog, emmenagogue, grammalogue, harpoon log, hearing dog, hunting dog, lymphagogue, maltese dog, melanagogue, prairie-dog, prairie dog, psychagogue, psychologue, ptyalagogue, ptysmagogue, raccoon dog, robber frog, rudraprayag, sausage dog, skittle-dog, spring frog, tailed frog, typedprolog, working dog

12 letters:
barking frog, carriage dog, cricket frog, delta-prolog, domestic dog, galactagogue, goliath frog, isotopologue, leopard frog, magyarorszag, memphremagog, phlegmagogue, secretagogue, shepherd dog, sporting dog

13 letters:
beetsterzwaag, cascades frog, modula-prolog, monopolylogue, pepe the frog, pickerel frog, systemverilog, yorkshire fog

14 letters:
admiralbulldog, andorra-prolog, antisialagogue, card catalogue, french bulldog, helminthagogue, seed catalogue

15 letters:
english bulldog, hair of the dog, jacob rees-mogg, multi-binprolog, parts catalogue, tarahumara frog, topdog/underdog

16 letters:
cape hunting dog, course catalogue

17 letters:
belgian sheep dog, chrysanthemum dog, german police dog

18 letters:
shetland sheep dog, sonic the hedgehog

19 letters:
african clawed frog, african hunting dog, chameleon tree frog, german shepherd dog

20 letters:
eastern cricket frog, japanese raccoon dog

21 letters:
northern cricket frog, whitetail prairie dog

23 letters:
south american bullfrog, tears in rain monologue

24 letters:
courage the cowardly dog

25 letters:
the princess and the frog