  Recent observations by James Webb Space Telescope discovered a number of
high-redshift galaxies with strong emission lines from doubly ionized oxygen.
Combined with ALMA observations of far-infrared lines, multi-line diagnostics
can be applied to the high-redshift galaxies in order to probe the physical
conditions of the inter-stellar medium. We study the formation and evolution of
galaxies using the FirstLight simulation suite, which provides outputs of 62
high-resolution, zoom-in galaxy simulations. We devise a physical model of HII
regions and calculate spatially resolved [OIII] line emission. We show that
massive galaxies with stellar masses of $M_* > 10^9 M_\odot$ chemically evolve
rapidly to $z=9$. Young stellar populations in the star-forming galaxies boost
the [OIII] line emission, rendering the ratio of line luminosity to star
formation rate larger than that for low-redshift galaxies, which is consistent
with recent observations. Measuring the flux ratios of rest-frame optical and
far-infrared lines allows us to estimate the physical conditions such as
density and metallicity of the star-forming gas in high-redshift [OIII]
emitters.
