Oh la la is a well known French expression of surprise, admiration or mild shock, and oui simply means yes, so the combined phrase works even for viewers who do not speak French. Short clips of the line have circulated on soundboards and in meme edits where a character needs an instant, exaggerated reaction. Its popularity comes from how recognisable the rhythm is, the drawn out ending turning a simple yes into a joke about being impressed, flustered or playfully scandalised by whatever just appeared on screen in the edit itself today.