one vs a

one

number, determiner

used with a person's name to show that the speaker does not know the person
synonym a certain

He worked as an assistant to one Jonathan Jones.

a

indefinite article

used before countable or singular nouns referring to people or things that have not already been mentioned

a man/horse/unit

an aunt/egg/hour/X-ray

2 used before uncountable nouns when these have an adjective in front of them, or phrase following them

a good knowledge of French

a sadness that won't go away

4 used to show that someone or something is a member of a group or profession

Their new car's a BMW.

She's a Buddhist.He's a teacher.

more info :http://oaadonline.oxfordlearnersdictionaries.com/dictionary/one