There are three main sources for the Quantity On Order:
1. Open Purchase Orders > when Items are purchased such as normal Inventory Items and Assembly Items when Allow Purchase of Assembly Items preference is enabled under Setup > Accounting > Accounting Preferences > Items/Transactions > Inventory.
2. Open Work Orders > for Assembly Items.
3. Transfer Orders > when Multi Location Inventory is enabled. The quantity will be added as On order in the 'To Location' set on the Transfer Order.
Note: Transactions that have Status: Pending Approval are excluded.