The word "one" doesn't begin with a vowel sound. Instead, it begins with a consonant "w" sound. Therefore, it needs the article "a," not "an."
Also asked, why an is not used before university?
If you observe, the 'U' in university has a consonant sound of 'yoo' or 'you'. So despite the fact that the word 'university' starts with a vowel, because of its consonant sound we use 'a' instead of 'an'.
Why Article A is used before university?
So, you need to remember that if a word begins with a vowel, but has an initial consonant sound, ('university' as an example), use the article 'a' before it. ABBREVIATIONS: The first thing to do is to figure out if the abbreviation is an acronym (pronounced as one word) or letter-by-letter.
Is it a universe or an universe?
Because universe, university, etc., although they are spelled with a vowel at the beginning, are not pronounced with a vowel at the beginning. The first sound of these words is a consonant, specifically a palatal approximant, which English speakers would usually call a "y sound."