The synergistic effect between displacement damage dose (DDD) and analog transient radiation effects on electronics (ATREE) in an operational amplifier (LM124) (opamp) from three different manufacturers is investigated. Pulsed X-ray experiments have highlighted ATREE sensitivity on devices significantly more important following exposure to fission neutrons than for unirradiated devices. A previously developed simulation tool is used to model ATREE responses taking into account the electrical parameters degradation due to displacement damage phenomenon. A good agreement is observed between model outputs and experimental ATREE results.