u/ALittlePeaceAndQuiet

In American English (I think in other dialects too), putting an h after a vowel tells us that we pronounce the short version of the vowel. It's so well-known that it's often used to explain pronunciation, with "ah," "eh," "uh," showing up in simple pronunciation guides. But O is the exception, given that "oh" uses the long version of the vowel. Why is that? I'm curious how h was first used with vowels in this way, and whether there are clear examples in the beginning of its use that explain this.