Opencart Option SKU Manager
Opencart do not assign unique SKU to each option of a product. For example, if one product has multiple options (sizes & color), then you cannot update the inventory of each option directly. You have to enter/scan product SKU, then select one option from option list. To remove this option selection step, we have developed a module (Option SKU Manager) to assign unique SKU to each option of product. After installing this module, you can directly enter/scan the SKU of option of SKU and update the quantity. You can download this module from here.
If there is a feature you want on your Opencart site, chances are that an Opencart plugin will allow you to do it, if you don't find any such plugin you can always contact us for your custom plugin development requirements.