Spoken grammar: lexico-grammatical features of conversation highlighted in corpus research.
Interjections, such as ‘oh’, are ‘inserts’ or ‘stand-alone’ words: they have a semantic rather than a grammatical link to the rest of the utterance. (See my entry 'I is for ... inserts.') Sometimes they may not be regarded as words at all, and spellings may vary: ‘yuk’ or ‘yuck’, ‘oops’ or ‘whoops’ (or is ‘whoops’ a different, slightly more emphatic word?), ‘brr’ – it’s cold – or ‘brrr' (it's colder?). But essentially, they express an immediate emotional response to a situation.
It can be interesting to consider the hundreds of words that can be defined as interjections, examining the difference, for example, between ‘ah’ expressing ‘I’ve just realised something’ and ‘aha’ meaning ‘suddenly I understand something which I didn’t understand before’. But, from a frequency point of view, ‘oh’ is overwhelmingly the most commonly used: 8,000 occurrences per million words in British English conversation, compared to less than 25 for ‘oops’ (Longman Spoken and Written English Corpus).
There are probably three reasons for its high-frequency rating:
1. The number of emotions it can express, including surprise, disappointment, understanding, anger, joy, desire, pain etc.
2. The ease with which it combines with partner words and phrases: oh dear, oh no, oh yeah, oh well, oh I see, oh really? oh look at that, oh gosh! (replace 'gosh' with your own swear/taboo word, as preferred), oh that’s amazing/terrible etc.
3. The fact that it can function as an interjection, a response form or a discourse marker (respectively in this dialogue):
A: I can’t find the tickets.
B: Oh no! The taxi will be here in a moment. When did you last see them?
A: I thought I gave them to you.
B: Oh. Are you sure? Because I haven’t got them now.
A: Oh, let’s not worry. They’ll have a record on their computer system.
To find out about teaching spoken grammar, you are welcome to visit my online course, Spoken Grammar: A Guide for English Language Teachers.