So here’s a double shot since I missed yesterday. (Sometimes I get busy. Sue me.)
Did they ever wear big wigs? I sure hope so.
Logically, I would explain that X’s sort of resemble someone with outstretched arms — like a hug — and O’s could represent an open mouth — a pretty big kiss, no doubt. According to Wikipedia however, X’s generally represent kisses and O’s the hugs, which kind of spoils my explanation.