When somebody's jaw always seems to be hanging down, mouth is always involuntarily open, etc. Douchebags, dumbasses and hoodrats make up most of the infected population with Open Mouth Syndrome.
Pat- "That douchebag TJ never closes his mouth. Always looks like he's open for business and deepthroat sessions. Guess he's just retarded."
Jon- "Yeah dude that dumbfuck has Open Mouth Syndrome."