I'm a lawyer in my day job, and one of the things I do when I read legal documents is look for what they say--not for what the author might have meant. It's hard to stop thinking like a lawyer, and I often find myself in that mental state when I'm reading game rules.
Looking at the Holmes version of the Dungeons & Dragons basic rulebook through the eyes of an attorney, I noticed something interesting--reading the rules as written, there are no express upper limits to ability scores!
Under CHARACTER CREATION, the book tells players to roll three 6-sided dice for each characteristic. Dr. Holmes then states, "18 is as high as one can get with three dice, so a character with a strength of 18 would be super-powerful, one with a strength of 3 (lowest possible dice roll) would barely be able to lift his sword off the ground."
No big deal, you think--the Good Doctor doesn't have to expressly limit your character's abilities, the dice and mathematics do it for him. However, take a look at the next section, ADJUSTING ABILITY SCORES.
Characters can raise their prime requisites by lowering their scores in other characteristics (for example, a fighting man can trade 2 points of intelligence (or 3 points of wisdom) to raise his strength by 1 point. And while the rules say you can't lower any ability below 9, the book does not impose an upper limit of any sort. So say you roll a character with an 18 for a prime requisite and some good scores in other abilities. Drop 2 or 3 points in some non-vital characteristics, and you can push that prime up to 19, 20, or even more depending on how high your other scores originally were.
You say that was an oversight; there's a hard maximum. But if Holmes meant ability scores to have an upper limit, he didn't share that with us. For example, the second-level magic-user spell Strength increases a fighter's by 2-8, a thief's by 1-6, or a cleric's by 1-4. Nothing in that description about up to 18.
The only mention of 18 as an upper limit for ability scores can be found, oddly enough, in the description for the Ring of Weakness. Describing the effects of how this magic item sometimes works in reverse, the text states a ring occasionally will increase the wearer's strength, "up to the maximum of 18, naturally." However, I could argue that this maximum only applies to the Ring of Weakness--otherwise the limit would have been mentioned elsewhere.
Of course, this is all pretty much a moot point, since most ability scores don't make that much difference mechanically in this set of rules. But still, by the book, Holmes basic D&D allows ability scores higher than 18.
DMs, what do you think? Would you allow 18+ ability scores it in your Holmes basic game?