ABSTRACT We report on detections with the Atacama Large Millimeter/sub-millimeter Array of the far-infrared [O iii] 88 μm line and the underlying dust continuum in two quasars in the reionization epoch, J205406.48−000514.8 (hereafter J2054−0005) at z = 6.0391 ± 0.0002 and J231038.88+185519.7 (hereafter J2310+1855) at z = 6.0035 ± 0.0007. The [O iii] luminosities of J2054−0005 and J2310+1855 are L[O iii] = (6.8 ± 0.6) × 109 and $(2.4 ± 0.6) \times 10^{9}\,$L⊙, corresponding to ∼0.05% and 0.01% of the total infrared luminosity, LTIR, respectively. Combining these [O iii] luminosities with [C ii] 158 μm luminosities in the literature, we find that J2054−0005 and J2310+1855 have [O iii]-to-[C ii] luminosity ratios of 2.1 ± 0.4 and 0.3 ± 0.1, respectively, the latter of which is the lowest among objects so far reported at z > 6. Combining [O iii] observations at z ≈ 6–9 from our study and the literature, we identify the [O iii] line deficit: objects with a larger LTIR (total infrared luminosity) have lower L[O iii]-to-LTIR ratios. Furthermore, we also find that the anti-correlation is shifted toward a higher LTIR value when compared with the local [O iii] line deficit.