As the semiconductor industry continues to scale down critical dimensions (CD), proximity effects get more and more severe. As such, aggressive Optical Proximity Correction (OPC) features like hammerheads, serifs and assist bars inevitably appear on fabricated masks. The great challenge, however -- to reliably assure the quality of these advanced masks -- is to be able to direclty judge a controversial defect under such complex features. It is necessary to find a more effective way to accurately disposition the defects found on these masks. Simulation-based defect disposition strategies have now become much more important for judging defect printabiilty. In this paper, we use variaous simulation tools to make a systematic study of defect printability right from the design to wafer printing. Four different combinations of OPC features with assist bars are presented here to demonstrate the defect printability and their induced CD changes compared to wafer results.