Remembering spellings of readings that have a long O

So, as I believe most students of Japanese are aware, there are two ways to spell the long O sound. Ō itself could be either おお or おう, could be either こお or こう, could be either そお or そう, etc. More often, though, the second kana is う, which sometimes throws me off when it’s お and I’m paying more attention to the mnemonic than the kana spelling—I’ve been penalized a couple times for misspelling 十日 as とうか instead of とおか.

For this specific example, since it’s affecting the kun’yomi of 十, what I’m trying to teach myself is that 十 in Arabic numerals is 10, the second digit of which looks like an O; thus the second kana should be お. Does anybody have any other suggestions on remembering the exceptions where the second kana for the long O is お instead of う, though?

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Well, more than that, long-o almost always ~おう. You can almost count the exceptions on one hand. Off the top of my head, 大きい, 多い, 通る, 氷, 凍る and 十 are all the ones on WaniKani (and also conjugations or compound words involving those). The only other common-ish one I can think of on top of those is 頬

So I don’t really have any suggestions or rules for remembering them beyond just “make a list, and check it twice”.


Yeah I just remember the おお exceptions manually.

It may help (or not) to understand those おお are not spelt like that just because some scribe wanted to introduce variety, but they where historically おほ (and the h sound faded away).
(While a lot of the おう - but not all - where originally あう. That is why お早う is おはよう、 polite form of Xい is おXうございます : はやい → はやう → はよう )