I've been watching a lot of discussions about this pop up lately, so I've decided to collect one final summary of my (eminently reasonable) interpretation of using Chainmail with Original Dungeons & Dragons. Examples of this system in play can be found here and here.
Summary of Terms and Rules
Scale: 1" = 10 yards (wilderness) or 10 feet (dungeon). 1 turn = 10 rounds = 10 minutes.
Melee: An exchange of blows between combatants within melee range (3") during a round of combat (1 minute).
Normal Men: Combatants (whether men or monsters) with fewer than two hit dice or equivalent fighting capability. By default, the fighting capability of a normal man is a single attack with no modifiers to hit.
Normal Combat: Melee where at least one side is comprised of normal men. Combatants make attacks according to their fighting capability, using Chainmail Man-to-Man combat. Normal men making attacks against fantastic combatants are reduced to one unmodified attack. Attacks against tough, squamous monsters will use the monster's armor class or the armor class of Horse: No Armor if it is superior. Unarmed monsters use small claws and teeth (dagger and hand axe), large claws and teeth (sword and battle axe) or large fists (mace).*
Fantastic Combatants: Combatants (whether men or monsters) with two or more hit dice or equivalent fighting capability.
Fighting Capability: The number of attacks a combatant may make against normal men, with any bonus added as a modifier to hit for a single attack in the series (i.e. 4+1). By default, a monster's fighting capability is equal to his hit dice (minimum 1+0). If a class has two fighting capabilities (i.e. the Swordsman), the heroic entry is used in fantasy combat and the other entry is used in normal combat against normal men.
Fantasy Combat: Melee where both sides are fantastic combatants. Player characters may either fight as normal men or make fantastic strikes. Other opponents make monstrous strikes.
Fantastic Strike: q.v.**
Monstrous Strike: A single attack as a normal man in normal combat, with an added bonus to hit equal to half of the attacker's hit dice (rounded down).**
Hit: A successful strike from normal combat, inflicting 1-6 points of damage.
* The last two lines of this rule have been added to supplement the normal rules with greater detail.
** I have deigned it necessary to add these rules to the corpus, see my reasoning in other articles. All other rules have been inferred directly from the text without modification.