ABSTRACT We study statistics of the 21-cm signal from the epoch of reionization. We propose to use [O iii] line emitting galaxies to cross-correlate with the 21-cm signal from z = 7–10. To this aim, we employ simulations of reionization obtained post-processing the high-resolution cosmological hydrodynamic simulation Massive Black-II with the three-dimensional (3D) radiative transfer code crash to follow the propagation of ionizing photons from a variety of sources. We show that, during the early phases of reionization, the 21-cm signal is positively correlated with the spatial distribution of the [O iii] emitters on large scales $(k \lt 1\, h~\rm cMpc^{-1})$. This positive correlation is generated by the temperature–galaxy correlation and it is a few times larger than when we assume that the heating is saturated. As the reionized regions expand, the correlation changes its sign to negative from z = 10 to 8. The signals at this epoch can be detected by combining the Square Kilometre Array (SKA) and a wide-field [O iii] emitter survey. We also calculate the cross-power spectrum with a 3D [O iii] intensity field, aiming at exploiting future intensity mapping observations. We conclude that high-redshift [O iii] line emitters can be used to probe the reionization process when the intergalactic medium is largely neutral.