There is a useful rule that says that, whenever you have the word one after an adjective, the word that carries the stress is the adjective and the pronoun one is deaccented. This is what happens in the following examples:
Would you like the RED one or the GREEN one?
My car broke down again. I’m going to buy a NEW one.
There are a few expressions, however, which don’t follow the rule. Against the general principle, in the phrase the only one, the stress is placed on one instead of only.
I was very lucky this morning because I chanced upon a very good example of this phenomenon on BBC4, which made my day. Now I can share it with you:
The only one is not the only case in which this happens. According to J.C. Wells (English Intonation, Cambridge. Great book!), the same accentuation pattern is found in the right one, the wrong one, the first one and the last one.
So, we would say:
She was the first ONE to see it. or
I don’t like that hat you’re wearing. You bought the wrong ONE.
Stress is a slippery matter sometimes indeed!