There are a couple of different ways that you can use a barcode scanner with stock control using OSOP.
The simplest way to do this would be to go into OSOP, open the product dialog box. Click Stock -> Add Stock
Tick the option that says use EAN/Barcode.
Click the box that says Product Identifier:
Then when you scan the barcode the EAN/ Barcode number should appear in that box.
You can then manually press find to show you the current stock and then type in the amount of stock to add in.
However you can make this into a more automated process by using a programmable barcode scanner to follow the manual steps given above.
Note: automatic stock update is only available in the Premium version of One Stop Order Processing