We present a catalogue of spectroscopically selected [O  III ]+H β emitters at 6.75 <  z  < 9.05 and the resulting [O  III ] 5008 Å luminosity function (LF) in the COSMOS field. We leveraged the 0.33 deg ² covered by COSMOS-3D using NIRCam/WFSS F444W to perform the largest spectroscopic search for [O  III ] emitters at 6.75 <  z  < 9.05. We present our catalogue of 249 [O  III ] emitters and their associated completeness function. The inferred constraints on the [O  III ] LF enabled us to characterise the knee of the [O  III ] 5008 Å LF, resulting in improved [O  III ] 5008 Å LF constraints at z  ∼ 7, 8. Notably, we find evidence for a decline of the [O  III ] luminosity density between z  ∼ 6 − 8, which could be expected if the metallicity of [O  III ] emitters, as well as the cosmic star-formation rate density, is declining at these redshifts. We find that theoretical models that reproduce the z  ∼ 7, 8 [O  III ] 5008 Å LF do not reproduce well the [O  III ] equivalent width distribution, pointing to potential challenges in the modelling of [O  III ] and other nebular lines in the early Universe. Finally, we provide the first constraints on the cosmic variance of [O  III ] emitters, estimating at 15% the fractional uncertainty for the z  ∼ 7, 8 [O  III ] 5008 Å LF in the 0.33 deg ² field. This estimate is in good agreement with that inferred from clustering, and it shows that the [O  III ] 5008 Å LF derived from smaller extragalactic legacy fields is strongly affected by cosmic variance. Our results highlight the fundamental role that wide-area JWST slitless surveys play to map the galaxy large-scale structure down into the reionisation era, serving as a springboard for a variety of science cases.