On order stock is the qty still to be delivered for the material for the all the P.O
you can see in ME2L enter the material and in the selection parameters select WE101
This will not have any effect on the stock overview in MMBE
In MMBE you can see all the status of the material
Stock on-hand means the stock you have physically in your WH.
Stock on-order is likely to mean the stock that is on order - i.e. you already orderd but it still it hasn't been dipatched from your vendor.
Compare this stock with all open PO qty (ME2M + selection parameter WE101 / or MD04).