Most words can be sounded phonetically. Linguistics researchers established that there are less than ten percent divergence or irregular spellings where it is impossible to apply the phonic sounds.
The word "one" is one of these holpelessly irregular words. Fortunately, "one" is such a common word that most kids would have learned it even before phonics is taught.
To learn irregular phonic words, start a stack of cards and call it "Sight Words". Review it using the 3 P Lesson Plan. Click here to learn about 3 Period Lesson