How quantities sync
The two available routes, and how to tell which one is yours.
From Odoo to your store
Quantity syncing runs one way: Odoo is the source of the balance and your store displays it. This is what stops you selling what you do not have.
Two routes, by hosting type
On Odoo.sh, syncing goes through the Tecfy addon installed inside Odoo.
On Odoo Online no addon can be installed, so the connector builds what it needs inside Odoo itself.
How to tell which is yours
Look at your sidebar: Products Quantities means Odoo.sh, Products Quantities (Online) means Odoo Online. The two never appear together.
Sidebar not updated yet?
The sidebar reads your hosting type, Odoo version and product-push settings from the session, loaded once when you sign in. After changing any of them, sign out and back in so the menu matches your new settings.
Stock is deducted when the delivery is validated
An order alone does not change your Odoo balance. The deduction happens when the stock picking is validated. If you never validate them, your quantities will not drop.