A first systematic study of [OIII] 88μm at z>8: two luminous oxygen lines and a powerful ionized outflow in the first 600 million years We present deep ALMA Band 7 observations of the [O iii] 88 µm line and underlying dust continuum emission in four UV-bright, gravitationally lensed (magnification µ = 1.4 − 3.8), JWST-selected galaxies at z = 8.5 − 10.3, with observed magnitudes −22.5 ≲ MUV ≲ −20.5. [O iii] 88 µm is confidently detected in UNCOVER-10646 at z = 8.5080 ± 0.0011 (15σ) and DHZ1 at z = 9.3113 ± 0.0006 (6σ), with both being intrinsically luminous systems [L[O iii] = (1.1− 1.6) ×109 L⊙] that follow the local [O iii]-SFR relation. [O iii] 88 µm remains undetected in the two z > 10 targets, including in the z = 10.07 X-ray AGN UHZ1, where we obtain a deep limit of L[O iii] < 6 × 107 L⊙. Dust emission is not detected in any individual source nor in a stack (< 3σ). The high S/N [O iii] 88 µm detection in UNCOVER-10646 uniquely reveals an additional broad component (FWHM = 1366+473 −329 km/s; ∆BIC ≈ 20) indicative of an ionized outflow. We infer a high outflow rate of M˙ out = 128+80 −46 M⊙ yr−1 , corresponding to a mass loading factor η = M˙ out/SFR = 2.9 +1.8 −1.0 that matches or exceeds theoretical predictions and JWST-based studies of ionized outflows at high redshift. While high-resolution ALMA follow-up is required to confirm and spatially resolve the outflow, this first systematic study at z > 8 highlights the unique diagnostic power of [O iii] 88 µm in characterizing galaxies in the early Universe. 9 Maynooth Academic Publishing