Microwave-assisted magnetic recording (MAMR)  is one candidate for next-generation perpendicular magnetic recording . Stable oscillation is one of the most important factors for spin-torque oscillators (STOs) used in a MAMR system. We performed micromagnetic simulations and found that stable STO oscillations were hard to obtain when the STO was inserted into the main pole - trailing shield (MP-TS) gap, primarily due to the strong magnetostatic interactions between the STO and write head , We also showed that the rise time of the field applied to an isolated STO greatly affected the STO oscillation , i.e. a shorter rise time gave better. more consistent STO oscillation. In this paper, we show that the rise time of the in-gap field acting on the STO is critical to stable STO oscillation. We also show that the combination of a tilted STO and a tilted main pole - trailing shield gap results in stable STO oscillation due to weaker magnetostatic interactions between the STO and write head. Calculation Model A micromagnetic model analysis was carried out considering a double-layered STO utilizing transmission spin torque. We used commercial micromagnetic software (Fujitsu, EXAMAG v.2.1) , The thickness of the field generation layer (FGL) was 10 nm, whilst the spin injection layer (SIL) was 2 nm thick. A 2 nm thick, non-magnetic inter layer was located between the FGL and SIL. The saturation magnetization (4πMs) was 20 kG for the FGL and 6 kG for the SIL. The anisotropy fields (Hk) of both the FGL and SIL were 31.4 Oe. The exchange constants, A, were 2.5×10-6 erg/cm3 for the FGL and 0.75×10-6 erg/cm3 for the SIL. The Gilbert damping factor, α, was 0.02 for both the FGL and SIL. The write head model had overall dimensions close to those of commercial write heads (3.25 μm × 2.55 μm × 4.5 μm).