$ grep UO ~/CSW15.txt | wc -l 603

$ grep OU ~/CSW15.txt | wc -l 12564

the most common are QUO- and -UOUS words, plus a handful of _UOS words like "virtuoso"; removing those we are left with a handful of root words:

FLUOR [as noted], BUOY [which americans pronounce "booee" and everyone else "boy", so the letter sequence even leads to dialect pronunciation differences!], DUO, LANGUOR [i've seen this misspelt a lot too], PLUOT, SKEUOMORPH, and a handful of scientific words like GLUON and VACUO-, and words borrowed from other languages like EUONYM and OCTUOR.