Hi Lianechu and welcome to GoY, where are you on the planet? Looks like too little water to me but there could be other factors depending on where you are... Would add that this is a shrub not a house plant.

Hi, welcome to GoY, do you know which species of Osmanthus you have, there are quite a few ?.
Looking at the photo I would say that you have Osmanthus decorus, but this usually flowers in mid spring, I notice you have it in a pot, and it looks as though it's inside somewhere, it's fully hardy and should be grown outdoors, if it is inside, it could be the reason the leaves are dropping, and why it's flowering now, if you are growing it inside, and it is O decorus, you would need to harden it off before planting outside in spring, Derek.

is outside but under a shelter/porch? it could well be in a 'wind tunnel' and that would cause the brown leaf tips. as well as lack of water as suggested.

Some forms of O. fragrans bloom in fall. That soil looks very dry, too, with maybe a cup or two of water added recently. I recommend that when you water, soak the the soil thoroughly, then let the top 1/2 inch dry out before you water again. Unless drainage is poor, you can't overwater container plants by putting too much water on at a time, only by watering too often.