OSS– dealing with the telecom network itself, supporting processes such as maintaining network inventory, provisioning services, configuring network components, and managing faults. (More Hardware Oriented.)
BSS – dealing with customers, supporting processes such as taking orders, processing bills, and collecting payments. (More Software Oriented.)
Of course, BSS and OSS is more complex than that. Usually different companies have different ways of distinguishing what is OSS and what is BSS.
Well explained in the video.