OD&D follows the Chainmail convention of using “inches” as a measure meaning 10 feet (in a dungeon) or 10 yards (outside). This isn’t explained up front: in Book 1, Men and Magic, measurements are just given in inches and you have to deal with it. I like to imagine the sad sap who didn’t have Chainmail and was figuring things out as he read. “Lightly-armored troops can travel 12 inches every ten minutes? Huh. Seems a little slow, but OK.” “The Light spell casts light in a 3 inch diameter? Wow. If it was radius, that would at least be something.”
The actual inches-to-distance conversions are somewhere in book 3. Underground movement is on page 8, in the Move/Turn in the Underworld section: “In the underworld all distances are in feet, so wherever distances are given convert them to tens of feet.” Outside movement is defined (inside parentheses) on page 17, under “Sighting Monsters”: “Players will see monsters at from 40-240 yards (inches convert to tens of yards for the wilderness) unless the monster has surprised the characters involved.”
The poor first-time OD&D reader won’t be skipping around to find this information either, because the Introduction admonishes the reader that volume 3 is presented last “in order to allow the reader to gain the perspective necessary – the understanding of the previous two booklets. Read through the entire work in the order presented before you attempt to play.”
My absolute favorite measurement-related section is the descriptions of the Wall of Stone and Wall of Iron spells (in Book 1, long before the meanings of “inch” are given), where inches (meaning tens of feet) interacts with feet (meaning feet), and also with inches meaning inches:
Wall of Stone: The creation of a stone wall two feet thick with a maximum length and height equalling 10 square inches. The wall will last until dispelled, broken down or battered through as a regular stone wall. Duration: 12 turns. Range: 6″.
Wall of Iron:: Like a Wall of Stone, but the thickness of the wall is three inches and its maximum area 5 square inches. Duration: 12 turns. Range: 6″.
“Huh. Wall of Iron produces 15 cubic inches of iron? Well, since my light footman moves a foot every 10 minutes, I guess it’s useful. How high do you think he can jump?”