You don't always get a warning message on invoices when you oversell an item in QuickBooks Online. To be notified if you are out of stock already, you'll need to set a reorder point. This process will give you alerts whenever the stocks are low or no longer available.
To do that:
Click on Sales at the left pane.
Hit on Products and Services.
Find the product you want to have a reorder point.
Select Edit in the Action column.
In the Reorder point field, add the threshold when you should reorder more stock.
Select Save and close.
After that, you can now check if you have low stock or out of stock items.
Here's how to do that:
Hit on Sales.
Go to Products and Services.
At the top, you’ll quickly see if you have low stock or out of stock items.
Select Low stock or Out of stock to see those products.