HO can mean and means so many different things to many people. For some it is talking about merge, to others it is about flying about nose to nose in any situation, to some it is a shot in any situation and maybe in about any frontal angle.
I personally understand it as a head on
pass (not related only to initial merge) where both planes can shoot at eachother at the same time, and as a
shot where one or both shoot at eachother during that previously mentione head on pass.
Maybe there may even be a head on
approach in a situation before the shots can be taken. However, if this approach is broken prematurely, there will be no head on pass or head on shot(s).
Also in my view there are always 2 planes in a head on approach and in head on pass. There may also be 2 head on shooters, but sometimes there is only one. In such case, if no prior rules were set, I'll just call the other one naive or adventurous. There is no point flying a head on pass if one has no intent on shooting... because that is the signal he is giving.
Like I've written before, I dont believe that anyone who is the 1st to point his plane at the enemy ever wants or wishes to "HO" the enemy. That option is left to the defending guy. He can choose to "HO" his attacker. Still it is most often these defenders who have then been shot down, who call their opponents "HOers"... go figure
Last but not least, "HO HO" said Santa Claus