This paper assimilates the in-situ O+ fluxes observations obtained from the Advanced Ionospheric Probe onboard the upcoming FORMOSAT-5 satellite and evaluates its possible impact on the ionospheric space weather forecast model. Observing system simulation experiment, designed for the global O+ fluxes, is shown to improve the electron density specification in the vicinity of satellite orbits. Root-Mean-Square-Error (RMSE) of the ionospheric electron density obtained from assimilating the daytime O+ fluxes can be improved by ~10% and ~5% for the forecast and nowcast, respectively. Although the improvement of nighttime O+ flux assimilation is less significant compared to the daytime assimilation, it still reveals impacts on the model result. This suggests that nighttime observations might not be sufficient to alter the trajectory of the model in the positive direction as the daytime result. Alternative data assimilation approach, such as assimilation of the empirical model built by the nighttime FS-5/AIP together with other existing satellite observations of O+ fluxes, could obtain a better accuracy of the electron density forecast.