Oh! Ow! Oy! Ooooooo! The letter o is the obvious choice for loud noises. It stands for an omnibus of attention-grabbing sounds, and its rotund, rotatable shape is also visually noticeable - in fact, it mimics the shape of your mouth as you pronounce the sound.
Like all vowels, o has a short sound (pot) and a long one, which may be summoned by leaving the word "open" (go, hello), or by adding a (boat), e (bone and toe) or w (snow).
Ow is well-known for its versatility: as well as the ow of snow, it's also the ow of cow. Less celebrated but even more versatile is ou - listen to it in cloud, could, country, cough, croup and colour. Indeed, I class ou (along with ea, ie and au) as one of the "danger digraphs" to which children should be alerted; each of these digraphs has a major sound value but none of them can really be trusted to make it.
The oo digraph also stands for two main sounds - as represented in boot and foot - but this presents less difficulty. Since children recognise and remember the oo shape very early (they see it looking back at them in a word like look), many of them don't even notice the sound variation.
Other common o digraphs are oioy (noisy, toys) and or (horse or doctor). But like every vowel, o crops up all over the place, representing all sorts of sounds - from the uh-sound of petrol and cotton to the i of women.