The optical characteristics of MoS2 monolayers on SiO2/Si substrates with an SiO2 thickness ranging from 40 to 130 nm are investigated. The measured Raman and optical reflection spectra of the MoS2 monolayers vary considerably depending on the SiO2 thickness. The Raman peak intensity of the MoS2 monolayer on the substrate with an 80 nm thick SiO2 layer is four times larger than those in the cases of 40- and 130 nm thick SiO2 layers, indicating a significant difference in the absorption at the excitation wavelength. The incident light undergoes anomalous phase changes upon reflection and transmission at the highly absorbing MoS2/nonabsorbing SiO2 or air interfaces. The phase changes at these interfaces in conjunction with those induced by the propagation of light in the SiO2 layer cause complex interference, which dramatically tunes the absorption spectrum of the MoS2 layer with changing SiO2 thickness. Neither wavelength nor the incident angle of light strongly affects the interface phase change. Thus, the MoS2 monolayers on sub-100 nm thick SiO2/Si substrates exhibit broadband omnidirectional absorption in the visible range. This work demonstrates that SiO2/Si wafers, which are the most popular substrates, allow the optical responses of MoS2 monolayers to be optimized for optoelectronic applications.