The OSS and IOSS returns can be used for reporting of B2C sales only. However, if you have also other transactions (B2B sales, movement of goods between warehouses or purchases) those have to be reported in standard VAT returns.
For example, movement of own goods between warehouses in different EU countries under FBA has to be reported in standard VAT returns and therefore standard VAT registrations are required in all countries where you keep their stock.