Hidden Rule #1
Missile Fire Ranges: OD&D lacks any information regarding missile ranges. Or does it? My first hidden rules in the annotations contained these:
Volume II: Monsters & Magic p. 10, Manticores; the rules state (...) with the range (18”) accuracy and effect of a crossbow.
The Nixies listing on p. 15 of the same volume gives javelins a 6” throwing range.
Volume III, Miscellaneous Weapons states the range for thrown axes, war hammers, and possibly spears (depending upon how one parses the description) a thrown range of 3”. All ranges are considered Medium with regard to modifiers, no Short or Long ranges.
Hidden Rule #2
Light Sources: as with missile ranges, nothing is stated explicitly in the section regarding illumination. With a bit of reading however, the light spells of the magic-user and cleric give us a starting point: cast light in a circle 3” in diameter. This is somewhat less than Dr. Holmes would give a torch in his Basic D&D "blue" book, he uses 3" radius, but it is a decent starting point.
Hidden Rule #3
Set Weapon versus Charge: under the entry for (magical) Spears we read [a]dditionally, if something impales itself upon the spear, damage will be double or even treble if the force is sufficient. This was often used as “set against charge” rule for any spear or polearm in many campaigns.
Hidden Rule #4
Partial Armor: how often does a monster strike at an unprotected head? On this page we find a guideline. It (the magic helm) does not protect in the same way as Magic Armor, so if it is worn in combat any hit upon its wearer should be given a 10% of striking the helm and smashing it (parenthetical comment added). With a little work this could be expanded into a house-rule for partial armors, if the referee so desired.
Hidden Rule #5
Gaze Attacks: the description for looking into this item (a mirror of life trapping) and falling victim to it gives a very good guideline for any gaze attacks: If it is openly displayed there is a 90% chance an unsuspecting creature will look full within it and be trapped. There is only a 10% chance a knowing person will be unable to avoid looking into it. Stumbling upon Medusa unprepared gives a 90% chance of meeting her gaze, if you’re ready you have a 1 in 10 of meeting her eyes and getting “stoned.”
Hidden Rule #6
How many adventurers can fight side-by-side in a 10' corridor? Using this rule can lead to some good PC tactics, such as using narrow passages to ameliorate a enemy's superior numbers.
Hidden Rule: most referees would allow that hobgoblins are roughly human sized (1+1 hit dice). In the second section of this entry, appearing on the next page, we read […] how many [hobgoblins] can come abreast down a typical passage in the dungeons? Allow perhaps 3 in a ten foot wide passage, and the balance will either be behind the front rank or fanning out to come upon the enemy by other routes. We used this as a basis for a 3-man front line in a party while in 10’ corridor. Accurate? It worked, and we still use that ruling to this day. But looking back, we wonder if 3 armed and armored fighting men side by side in a 10’ corridor would actually work at all.
Hidden Rule #7
Rowed Ships On The Open Seas: a nearly hidden rule allows the use of Viking longships on the open sea, as opposed to other types of rowed vessels because they were pierced high on the freeboard.