In this study, we examined the low-frequency spectra of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF4]) mixtures with methanol (MeOH), acetonitrile (MeCN), and dimethyl sulfoxide (DMSO), which were obtained by femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and molecular dynamics (MD) simulations. In addition, we estimated the liquid properties of the mixtures, such as density rho, surface tension gamma, viscosity eta, and electrical conductivity sigma. The line shapes of the low-frequency Kerr spectra of the three [MOIm][BF4] mixture systems strongly depend on the mole fraction of the molecular liquid, X-ML. The spectral intensity increases with increasing X-ML of the [MOIm][BF4]/MeCN system but decreases for the [MOIm][BF4]/MeOH and [MOIm][BF4]/DMSO systems. These behaviors of the spectral intensities reasonably agree with the vibrational density-of-states spectra when the polarizability anisotropies of MeOH, MeCN, DMSO, and ion species are considered. The characteristic frequencies (first moments, M-1) of the low-frequency spectra of the three mixture systems are almost insensitive at X-ML = 0-0.6. However, the frequencies vary mildly at X-ML = 0.6-0.9 and dramatically at X-ML = 0.9-1. The X-ML-dependent M-1 in the Kerr spectra are well reproduced by the MD simulations. Plots of M-1 versus bulk parameter, (gamma/rho)(1/2), for the three mixture systems show that the mixtures at X-ML = 0-0.6 behave like aromatic cation-based ionic liquids (ILs), those at X-ML = 0.9-1 are molecular liquids (MLs), and those at X-ML = 0.6-0.9 are transitioning between aromatic cation-based ILs and MLs. MD simulations show that the solvent molecules localized at the interface between the ionic and the alkyl group regions without forming large solvent networks at X-ML = 0-0.6. However, solvent networks or regions develop largely at X-ML = 0.6-0.9 and the constituent ions of the IL disperse in the MLs at X-ML = 0.9-1. The MD simulations corroborate the results obtained by fs-RIKES.