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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 photospherictemperatures with time, the optical colours show an apparent temperatureincrease from $\sim$15000 K to $\sim$20000 K over the first 70 days, likelycaused by an additional pseudo-continuum in the spectrum. Remarkably, thespectrum displays a forbidden emission line even during the rising phase of thelight curve, inconsistent with an apparently compact photosphere. We show thatthis early feature is [O II] $\lambda\lambda$7320,7330. We also see evidencefor [O III] $\lambda\lambda$4959, 5007, and [O III] $\lambda$4363 furtherstrengthening this line identification. Comparing with models for nebularemission, we find that the oxygen line fluxes and ratios can be reproduced with$\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 earlyonset of the emission lines requires that this material was ejected within thefinal months before the terminal explosion, consistent with the timing of theprecursor plateau. Interaction with denser material closer to the explosionlikely produced the pseudo-continuum bluewards of $\sim$5500 \AA. We suggestthat this event is one of the best candidates to date for a pulsationalpair-instability ejection, with early pulses providing the low density materialneeded for the forbidden emission line, and collisions between the final shellsof ejected material producing the pre-explosion plateau.

A Precursor Plateau and Pre-Maximum [O II] Emission in the Superluminous SN2019szu: A Pulsational Pair-Instability Candidate