Mammoth Uncensored Word List:
19 - 15 letter words starting with do

doctrinarianism documentarising documentarizing documentational dodecasyllables dolichocephalic dolichosauruses dolomitisations dolomitizations domineeringness dorsibranchiate dorsiventrality dorsoventrality doublebarrelled doubtlessnesses downheartedness downregulations downrightnesses downtroddenness