  With the Atacama Large Millimeter/sub-millimeter Array (ALMA), we report
detections of the far-infrared (FIR) [OIII] 88 $\mu$m line and the underlying
dust continuum in the two quasars in the reionization epoch,
J205406.48-000514.8 (hereafter J2054-0005) at $z=6.0391\pm0.0002$ and
J231038.88+185519.7 (hereafter J2310+1855) at $z=6.0035\pm0.0007$. The [OIII]
luminosity of J2054-0005 and J2310+1855 are $L_{\rm [OIII]} = (6.8\pm0.6)
\times 10^{9}$ and $(2.4\pm0.6) \times 10^{9}$ $L_{\rm \odot}$, corresponding
to $\approx$ 0.05\%\ and 0.01\%\ of the total infrared luminosity, $L_{\rm
TIR}$, respectively. Combining these [OIII] luminosities with [CII] 158 $\mu$m
luminosities in the literature, we find that J2054-0005 and J2310+1855 have the
[OIII]-to-[CII] luminosity ratio of $2.1\pm0.4$ and $0.3\pm0.1$, respectively,
the latter of which is the lowest among objects so far reported at $z>6$.
Combining [OIII] observations at $z\approx6-9$ from our study and the
literature, we identify the [OIII] line deficit: objects with larger $L_{\rm
TIR}$ have lower $L_{\rm [OIII]}$-to-$L_{\rm TIR}$ ratios. Furthermore, we also
find that the anti-correlation is shifted toward higher $L_{\rm TIR}$ value
when compared to the local [OIII] line deficit.
