1. SAP S/4HANA Product Assistance
"Stock of Material Provided to Vendor": In the section on Subcontracting within Inventory Management
the documentation states: "This special stock has the following features: It is part of your own valuated stock. It is managed at plant level because the stock is not stored in your company but on the vendor's site." (Path: SAP S/4HANA -> Product Assistance -> Enterprise Business Applications -> Sourcing and Procurement -> Inventory -> Subcontracting -> Stock of Material Provided to Vendor).
2. SAP Learning
Course S4525 "Inventory Management and Physical Inventory in SAP S/4HANA": Unit 5
"Special Forms of Stock"
explains that special stock 'O' (Stock of material provided to vendor) is a type of company-owned special stock that is managed at the plant level per vendor
not at the storage location level.