The questions in the title
One explanation I’ve heard is that it represents two pairs of lips touching:
On the left: >
On the right: <
Approaching: > <
Kissing: X
Pronounce it: eks. Sounds somewhat like kiss, specially if you don’t emphasize the vowel.
Over on "How Stuff Works," they have this to say:
Seems plausable to me.
See, it doesn’t seem very plausible to me because the first time I and my non-US classmates (from Argentina, China, Colombia, France, Germany and India among others) had seen exes and ohs in a signature was in the US and on the internet. Maybe it was a mixture of both things, but English-language only? I was familiar with signing with an X if you couldn’t sign your name, but that would be for official papers: if it was a personal letter (either written by yourself or by a scribe), the signature was your name.
Does anybody know of any non-Anglo cultures where X meant kiss, pre-internet?
All this time I thought X (crossed arms) was a hug and O (puckered lips) was a kiss.
Ignorance fought.
+1
And, FTR, I first saw ‘exes and ohs’ in the early '90s in a paper letter.
I always saw it as first part of a progression – start with x, end with xxx. I assumed xx would be heavy petting, or something. 
It goes back further than that. I can remember it being used in elementary school in 1983. The kids in my class were a little compulsive about making sure the number of XO’s was directly proportional to how much they liked you, leading to silly notes like “I had fun at your birthday party.” with XOXOXOXOXOXOXO filling up the remainder of the front of the page and all of the back. You knew the relationship was deteriorating if only the front of the page was filled up…
I can remember getting letters with XOXOXO from my grandmother, in the 1950’s.
I would guess it was one of the first emoticons.
There’s some thoughts on the subject in this thread on Domebo.
Nothing definitive, though. I’m a little bit skeptical of the signature explanation, but I’m pretty much skeptical of every explanation offered so far, although the two lips touching does seem more plausible than the other ones, to me.
I’m surprised Cecil has not answered this, actually.
What languages/cultures use this?
English in America
Others?
It looks like a pair of puckered lips.
Yes, around here (Quebec) it does.
Which lips are we talking about here?
XXX :dubious:
Illiterates in the UK used to sign “X” instead of a signature, and the X was then witnessed.
No idea how that relates, just a datapoint.
And yes it’s totally ubiquitous over here, and Ireland too.
(I have recently been corresponding with a girl I like - a lot - and due to this habit, today inadvertantly signed a business email with “Jim x”. Thankfully it was to a coworker with a sense of humour…)
I remember it from the 50s too. Among kids, though, not from from my grandmother.
I always knew X = kiss and O = hug, but I never knew why. X kind of looks right for a kiss, but O for a hug? 
I’m sure we’ve done this before, but I can’t remember if I ever found out.
I do remember calling my mom, and she said they did it in the late 1930’s. That’s all I can help with now.