  We study statistics of the 21-cm signal from the epoch of reionization. We
propose to use [OIII] 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 [OIII]
emitters on large scales $(k < 1h~\rm cMpc^{-1})$. 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 [OIII] emitter survey. We also calculate the cross-power
spectrum with a 3D [OIII] intensity field, aiming at exploiting future
intensity mapping observations. We conclude that high-redshift [OIII] line
emitters can be used to probe the reionization process when the intergalactic
medium is largely neutral.
