In English, people don't like to repeat the same noun in a sentence. You can replace the second noun with "one" and "ones." It helps your sentences sound more natural. Use "one" to replace a singular noun and use "ones" to replace a plural noun. I like the blue dress, but I don't want the green dress. = I like the blue dress, but I don't want the green ONE. These sunglasses are OK, but I want new sunglasses. = These sunglasses are OK, but I want new ONES.

One - a single item, a single person One's - somethng possessed by someone or something. Used when you would like to specify the ownership linkage between the person and the object (physical, or otherwise) in question "I would like to have one piece of fruit please." "When does one begin this test?" "One's piece of fruit should be placed on this plate." "One's test answers should be in pencil." Typically, when we use "one" to identify a person, we generally do not know who the specific person is. Sometimes we do, and if it is used when we know the person, there can be a hint of annoyance with that person. "Where should I put this suitcase?" "You should put that suitcase in the closet." "Where should I put this suitcase?" "One should put suitcases in the closet." In the second response, the person is really saying "You don't know where to put a suitcase????? Everybody knows they go in the closet."

It has to do with singular and plural. I’ll take that one! What’s wrong with those ones? Which ones are best? This one is the best! We are the only ones left from the training class. I am the only one left from the training class.

I assume you mean the pronoun One and One's? One is used similar to he/she but is not gender specific. For example "One cannot help oneself" (oneself is the reflexive version of one) or "One could argue" which is used a lot in essays to avoid use of the third person. Ones makes it possessive. So "That is just one's opinion" means that is just my opinion. If you meant one as in there is one green apple and 5 red ones, then ones is simply a plural of one meaning 5 of that type of apple.