1650s, from French oscillation, from Latin oscillationem (nominative oscillatio), noun of action from past participle stem of oscillare "to swing," supposed to be from oscillum "little face," literally "little mouth," a mask of open-mouthed Bacchus hung up in vineyards as a charm (the sense evolution would be via the notion of "swing in the breeze"); from PIE *os- "mouth" (see oral).