When using the Open Commerce API (OCAPI) to check product inventory across all inventory lists, the developer needs to ensure that the 'inventory_list' resource is enabled in the OCAPI settings. This setting enables the access and management of inventory data via the API. The inventory_list resource is crucial for performing operations related to inventory lists, such as retrieving inventory levels, updating inventory information, or querying multiple inventory lists simultaneously. Ensuring this resource is enabled is necessary for the API to perform inventory-related operations successfully.