Pronouns - one, ones

We use one (singular) and ones (plural) to avoid unnecessary repetition.

See those two girls? Helen is the tall one and Jane is the short one.
Which is your car, the red one or the blue one?
My trousers are torn. I need some new ones.
See those two girls. Helen is the one on the left.
Let’s look at the photographs. The ones you took in Paris.

We often use them after Which ... in questions:

You can borrow a book. Which one do you want?
There are lots of books here. Which ones are yours?

The Uses of One
As a determiner, the word one is sometimes used before a proper noun to designate, particularly, this person: "He delivered the package to one Ronald Pepin of Colchester." The article "a" will also function in that position for the same purpose.

Sometimes we use the word one as an adjective, as in "I'll have just one scoop of ice-cream," and we seldom have trouble with that usage. But we also use one as a pronoun, and this is where one becomes surprisingly complex.

Sometimes the pronoun one functions as a numerical expression:

Those are lovely scarves. I think I'll buy one.
One is hardly enough.
One is purple, the other green.
The three brothers get along quite well; in fact they adore one another.
One of the senators will lead the group to the front of the capitol.
The yellow car is fast, but I think the blue one will win.

The Plural of One
As a singular numerical pronoun, we don't have trouble with one: "Those donuts look delicious; I think I'll pick this one." But what if I want two donuts? It is possible, sometimes, to pluralize one:

I really like the chocolate ones.
The ones with chocolate frosting have cream fillings.
Are these the ones you want?
Do you want these ones?