Recently, the next generation (5-th generation: 5G) mobile communication systems have been actively investigated all over the world, in order to satisfy the strong demand for the faster and larger data communication. In 5G systems, not only UHF band but also SHF/EHF bands have been attracted much attention. Hence, frequency characteristics of radio propagation path loss have been studied. We have previously investigated outdoor to indoor (O2I) propagation characteristics in the 2.2 GHz band, and a modified path loss model instead of 3GPP model has been proposed. In this paper, in order to investigate frequency characteristics in terms of O2I propagation, the received power is measured for O2I propagation when considering 0.8, 2.2 and 4.7 GHz bands. Moreover, we investigate O2I propagation characteristics for different frequency bands by using O2I factor which denotes co-efficient versus transmit distance with logarithm expression when considering O2I propagation. Moreover, we consider O2I factor which is calculated by the least square approximation.