I do not want to turn away for fear of giving the red guy my six.
In time, you will learn to start your "turn(s)" before you actually merge. This is called a lead turn. If you set up your lead turn correctly, and at the right time, you will never be in a nose-to-nose merge. Now ... if I set up a real crappy lead turn and I get raked from nose to tail, as we merge ... not an HO as far as I am concerned. When I try to setup a lead turn, I want to create what I consider a good angle to be in at the merge (for a reversal) AND at the same time, not present myself as a good target.
As Ack Ack pointed out, when I see a guy coming at me, I don't vector my plane directly towards his nose. I start the separation (thinking lead tunr) by dropping my nose (to pick up a little extra speed) and then banking either right or left ... this is the vertical and horizontal separation that Ack Ack mentioned.
Now, as I make those adjustments, I watch his reaction and make similar adjustments as he gets closer. This is all being done to create my lead turn and if I have done it right and the other guy is still pressing to get a nose to nose solution, he will be dead within 1 or 2 turns, if he decides to stick around.
As mentioned above, if you come upon a guy that is not pressing the nose-to-nose merge, tighten your seat belt 'cause your in for a real fight.
Ack Ack has defined the "technical" head on ... others are talking about the "intentioned" head on. When I hear HO, I always think of the technical definition. It seems that about 98% of the time, I run into the intentioned head on ... guns blazing from D800 on it ... but I don't consider, when we merge, it to be a HO merge because I haven't given them my nose and I don't usually have a guns solution on them due to my angle for a lead turn.