Method and system for performance assurance in a network slice subset instance (NSSI) or a network slice instance (NSI) of a network. The method comprises receiving, at a network management function (OAM) of the network, a trigger indicating a network performance deficiency; based on the trigger, determining, by a data analytics (DAM) function of the OAM (OAM DAM) in coordination with a network analytics function of one of the core network and a radio access network (RAN), an NSI/NSSI modification; and implementing, by the OAM, a change in at least one of: NSI/NSSI policies, configurations in at least one of core network functions, the RAN and network resources, in accordance with the NSF NSSI modification.