Thank you for your prompt response, @justonwo.

I'd like to add more information about how QuickBooks Online tracks inventory. 

Purchasing and selling products after switching from QuickBooks Desktop to QuickBooks Online affects inventory numbers. As a result, it is vital to set up your inventory following the initial conversion.

The items in your inventory will be imported into the Products and Services list. You may notice that your imported items have sub-products called Inventory Asset and COGS. This is due to QuickBooks Online efforts to maintain an accurate accounting of asset value and Cost of Goods Sold.

Furthermore, QuickBooks Online Recalculates the inventory to First In First Out. Please see this page for more details: Move your inventory from QuickBooks Desktop to QuickBooks Online.

I'm also adding this article for your guide on how to set up inventory after converting from QuickBooks Desktop to QuickBooks Online: Set up inventory after conversion. It also includes steps on how to Offset the old inventory asset account. 

Please feel free to leave a reply on this thread if you have any additional questions about inventory adjustments. I'll be happy to help.