We present a study of overdensities identified with [O II] emitters at z = 1.47 selected based on the narrow-band images taken with the Hyper Suprime-Cam on the Subaru Telescope. We find 17 overdensities with statistical significance of ${\gt}4\sigma$ within a total area of 2.8 deg$^2$ in COSMOS and SXDS-XMMLSS fields. We estimate the total halo masses of our overdensities using two stellar-mass-based methods, indicating that our overdensities reside in group- to cluster-scale halos. Most of these overdensity regions are dominated by star-forming galaxies (emission-line galaxies), unlike local clusters, which are dominated by red quiescent galaxies. These systems therefore represent a new class of galaxy clusters/groups still in a young evolutionary stage, which would have been missed by conventional cluster finders that search for overdensities of old red-sequence galaxies. Star-forming galaxies in these overdensity regions show no clear differences from those in the general field in stellar mass ($M_\ast$), star formation rate (SFR), specific SFR (sSFR), or deviation from the star-forming main sequence ($\Delta$MS). We also find that emission-line galaxies with red colors, which likely include dusty star-forming galaxies, AGNs, and quenching galaxies, tend to reside in overdensity regions. The spatial concentration of emitters is comparable to that in the confirmed cluster XMMXCS J2215.9-1738 at z = 1.46. Finally, the giant-to-dwarf ratio (the number of giant galaxies divided by that of dwarf galaxies) for red-sequence galaxies does not show any significant environmental dependence. In contrast, the emitter GDR is enhanced in the overdensities, particularly in the more evolved systems, suggesting that massive star-forming galaxies preferentially inhabit dense environments. Overall, our overdensities selected with [O II] emitters are still at an early evolutionary stage of cluster/group growth with higher star formation activities, and environmental effects are not yet the primary driver of galaxy quenching.