  JWST has now made it possible to probe the rest-frame optical line emission
of high-redshift galaxies extending to z~9, and potentially beyond. To aid in
the interpretation of these emerging constraints, in this work we explore
predictions for [OIII] emission in high-redshift galaxies using the First Light
and Reionisation Epoch Simulations (FLARES). We produce predictions for the
[OIII] luminosity function, its correlation with the UV luminosity, and the
distribution of equivalent widths (EWs). We also explore how the [OIII] EW
correlates with physical properties including specific star formation rate,
metallicity, and dust attenuation. Our predictions are largely consistent with
recent observational constraints on the luminosity function, average equivalent
widths, and line ratios. However, they fail to reproduce the observed tail of
high-EW sources and the number density of extreme line emitters. Possibilities
to explain these discrepancies include an additional source of ionising photons
and/or greater stochasticity in star formation in the model or photometric
scatter and/or bias in the observations. With JWST now rapidly building larger
samples and a wider range of emission lines the answer to this remaining
discrepancy should be available imminently.
