To thoroughly investigate how to effectively enhance the energy harvesting efficiency of fluid-induced vibrations (FIV) in blunt body oscillators under low flow velocity conditions, a numerical simulation approach was employed. The FIV characteristics of rigid blunt body oscillators with varying thicknesses at Reynolds numbers (Re) ranging from 5×10⁴ to 1.4×10⁵ were examined. The analysis focused on the oscillators' amplitude, frequency response, lift mode, and flow field characteristics. The results demonstrate that all oscillators can achieve coupled vortex-induced vibration and galloping. Within the galloping branch, the FIV response and lift coefficient of the oscillators exhibit high sensitivity to variations in oscillator thickness. Notably, the medium-thickness oscillator undergoes a sudden amplitude jump and a sharp frequency drop at specific Re. The revealed intrinsic relationship among the FIV response, lift mode, and transient flow field provides valuable insights for the optimal design of oscillator structures in FIV energy harvesting applications.