This article presents the design of a security system for highfrequency oscillations in isolated and weakly interconnected power systems. The security system operates online and is based on two decision algorithms, which evaluate the system instability risk and determine the generating units and load circuits to be opened after a disturbance. In order to determine the generator tripping and load shedding combinations, the topology and the operating point of the weakly interconnected power system is considered. The decision algorithms were developed combining artificial intelligence and sequential algorithms techniques. The obtained tripping and shedding combination were simulated in DIG SILENT, where an isolated and weakly interconnected power system was modeled.