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The flux of the [O  iii ]λ5007‐Å line is considered to be a good indicator of the bolometric emission of quasars. The observed continuum emission from the accretion disc should instead be strongly dependent on the inclination angle θ between the disc axis and the line of sight. Based on this, the equivalent width (EW) of [O  iii ] should provide a direct measure of θ. Here we analyse the distribution of EW([O  iii ]) in a sample of ∼6000 Sloan Digital Sky Survey quasars, and find that it can be accurately reproduced assuming a relatively small intrinsic scatter and a random distribution of inclination angles. This result has several implications: (1) it is a direct proof of the disc‐like emission of the optical continuum of quasars; (2) the value of EW([O  iii ]) can be used as a proxy of the inclination, to correct the measured continuum emission and then estimate the bolometric luminosity of quasars and (3) the presence of almost edge‐on discs among broad‐line quasars implies that the accretion disc is not aligned with the circumnuclear absorber, and/or that the covering fraction of the latter is rather small. Finally, we show that a similar analysis of EW distributions of broad lines (Hβ, Mg  ii  and C  iv ) provides no evidence of inclination effects, suggesting a disc‐like geometry of the broad emission line region.

[O iii ] equivalent width and orientation effects in quasars