The Nature of High ${\rm [OIII]}_{\rm 88\mu m}$/${\rm [CII]}_{\rm 158\mu
  m}$ Galaxies in the Epoch of Reionization: Low Carbon Abundance and a
  Top-Heavy IMF?
  ALMA observations of $z>6$ galaxies have revealed abnormally high
[OIII]$_{\rm 88\mu m}$/[CII]$_{\rm 158\mu m}$ ratios and [CII]$_{\rm 158\mu m}$
deficits compared to local galaxies. Numerous solutions have been proposed
including differences in C and O abundance ratios, observational bias, and
differences in ISM properties. In order to elucidate the underlying physics
that drives this high-redshift phenomenon, we employ SPHINX$^{20}$, a
state-of-the-art, cosmological radiation-hydrodynamics simulation, that
resolves detailed ISM properties of thousands of galaxies in the epoch of
reionization. We find that the observed $z>6$ [OIII]-SFR and [CII]-SFR
relations can only be reproduced when the C/O abundance ratio is $\sim8\times$
lower than Solar and the total metal production is $\sim4\times$ higher than
that of a Kroupa IMF. This implies that high-redshift galaxies are potentially
primarily enriched by low-metallicity core-collapse supernovae with a more
top-heavy IMF. As AGB stars and type-Ia supernova begin to contribute to the
galaxy metallicity, both the [CII]-SFR and [CII] luminosity functions are
predicted to converge to observed values at $z\sim4.5$. While we demonstrate
that ionisation parameter, LyC escape fraction, ISM gas density, and CMB
attenuation all drive galaxies towards higher [OIII]/[CII], observed values at
$z>6$ can only be reproduced with substantially lower C/O abundances compared
to Solar. The combination of [CII] and [OIII] can be used to predict the values
of ionisation parameter, ISM gas density, and LyC escape fraction and we
provide estimates of these quantities for nine observed $z>6$ galaxies.
Finally, we demonstrate that [OI]$_{\rm 63\mu m}$ can be used as a replacement
for [CII] in high-redshift galaxies where [CII] is unobserved and argue that
more observation time should be used to target [OI] at $z>6$. (Abridged)
