This paper discusses the impacts of oxidizer-to-fuel mass ratio ($O/F$) shifts on the flight performance of single-stage sounding rockets using the flight simulations of three scales of $O/F$-controlled and $O/F$-uncontrolled hybrid rockets under a nominal fuel regression behavior without uncertainty. The flight simulation code includes three factors dependent on the $O/F$: thermodynamic states of the burned gas (theoretical $I_{\mathrm{sp}}$), shifts in $c^{*}$ efficiency, and nozzle throat erosion. In the flight simulations, a thrust control law was applied to increase the apogee and evaluate the effects of $O/F$ shifts in the thrust curve including throttling. For the best cases in each scale, $O/F$-controlled hybrid rockets slightly improved the performance by 2.03–2.42% in the averaged specific impulse. However, the performance of the $O/F$-controlled sounding rockets is essentially the same as the $O/F$-uncontrolled type under the median regression behavior: especially when considering the slight increases in the mass and complexity of the oxidizer feed system needed for $O/F$ control. Considerable scale effects on the throat erosion and theoretical $I_{\mathrm{sp}}$ were observed, but that of the $c^{*}$ efficiency was negligible. The improvement of the theoretical $I_{\mathrm{sp}}$ was the primary contributor to flight performance, which was responsible for a larger than 70% share in the total $I_{\mathrm{sp}}$ increase. The second largest contribution was the improvement of the $c^{*}$ efficiency with a share of 21.8–24.3%. The $O/F$ control gave an improvement of throat erosion corresponding to 5.75% in the total $I_{\mathrm{sp}}$ increase for the smallest scale; but, with increasing of the scale, the throat area increase ratio became small so that the throat erosion improvement contribution was reduced to 1.21%.