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My textbook gives the expression ''Oh!'' as ''あっ!'''.

I find this strange as little tsu makes the following consonant a double consonant.

What is it doing ''phonetically'' here?

If it is supposed to be extending the あ then should it not be ああ ?

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The half-size tsu (っ) in this case is introducing a 'glottal stop' (see here). You are correct that the half-size tsu is usually a feature of sokuon (marking geminate consonants) but it does have other functions too, as in this case.

Basically, this sounds like a sudden stop in the word. It gives the phrase あっ! a kind of immediacy which has the effect of inserting emotion into it. In that sense, it is similar to the English 'Oh!' in terms of effect (interjection), even though the phrases are not similar phonetically. It can sometimes express things like 'Wow" or "Oh no!", but it depends on the context.

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Depending on which kind of “Oh”, it can express...

[Surprised to find something: ヘェ] [Suddenly thought of something: あっ] [Figuring out something, finally: ああ] (to the best of my limited knowledge)

Just like in English there can be different renditions of “Oh” depending on the emotions.

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