The fragrance of osmanthus is mainly the volatile oils contained in the flowers. When you dry it out, all the volatile oils have evaporated, so there is no more fragrance. There is no good way to retain the aroma. You can put the flowers and tea together so that the tea will absorb the aroma. Another thing is to make osmanthus cake, or osmanthus honey. It's also very good. There are a lot of specific ways to do this on the internet. There are not enough words here so I won't introduce them all. I hope that one will be helpful to you