Microwave-assisted magnetic recording (MAMR) has been proposed as a candidate technology to realize areal recording densities of over 2 Tbit/inch². MAMR requires a spin-torque oscillator (STO) to generate a strong high-frequency magnetic field that will induce magnetic resonance in the recording medium. The oscillation characteristics of STOs were previously investigated using a micromagnetic model that neglected the magnetic interaction among the STO, the single-pole-type (SPT) writer, and the recording head. The STO is typically placed in the gap between the main pole and the trailing shield of the SPT writer, so that the STO is inevitably subjected to strong magnetic interaction with the main pole and the trailing shield. We have developed a new simulator, referred to as an integrated MAMR simulator, that takes this interaction into account. The integrated simulator has revealed that the magnetic interaction has a strong influence on the oscillation characteristics.