  We present a detailed study on SN2019szu, a Type I superluminous supernova at
$z=0.213$, that displayed unique photometric and spectroscopic properties.
Pan-STARRS and ZTF forced photometry shows a pre-explosion plateau lasting
$\sim$ 40 days. Unlike other SLSNe that show decreasing photospheric
temperatures with time, the optical colours show an apparent temperature
increase from $\sim$15000 K to $\sim$20000 K over the first 70 days, likely
caused by an additional pseudo-continuum in the spectrum. Remarkably, the
spectrum displays a forbidden emission line even during the rising phase of the
light curve, inconsistent with an apparently compact photosphere. We show that
this early feature is [O II] $\lambda\lambda$7320,7330. We also see evidence
for [O III] $\lambda\lambda$4959, 5007, and [O III] $\lambda$4363 further
strengthening this line identification. Comparing with models for nebular
emission, we find that the oxygen line fluxes and ratios can be reproduced with
$\sim$0.25 M$_{\odot}$ of oxygen rich material with a density of $\sim10^{-15}
\rm{g cm}^{-3}$. The low density suggests a circumstellar origin, but the early
onset of the emission lines requires that this material was ejected within the
final months before the terminal explosion, consistent with the timing of the
precursor plateau. Interaction with denser material closer to the explosion
likely produced the pseudo-continuum bluewards of $\sim$5500 \AA. We suggest
that this event is one of the best candidates to date for a pulsational
pair-instability ejection, with early pulses providing the low density material
needed for the forbidden emission line, and collisions between the final shells
of ejected material producing the pre-explosion plateau.
