I don't know why you want to grow osmanthus from seed. It will take years to flower. Why not propagate it from cuttings?

Here is one link, which tells you when and how to do it.

However, if you're adamant to have your plants produce seeds, you can try using cement powder. I haven't done it and I'm not sure if it's applicable to Osmanthus.

Good Luck.

Here is a link that might be useful: Producing Seeds with Self-Sterile Cacti