We study the formation of photospheric emission lines in O stars and show that the rectangular profiles, sometimes double peaked, that are observed for some stars are a direct consequence of rotation, and it is unnecessary to invoke an enhanced density structure in the equatorial regions. Emission lines, such as N iv λ4058 and the N iii λλ4634–4640–4642 multiplet, exhibit non-standard ‘limb-darkening’ laws. The lines can be in absorption for rays striking the centre of the star and in emission for rays near the limb. Weak features in the flux spectrum do not necessarily indicate an intrinsically weak feature – instead the feature can be weak because of cancellation between absorption in ‘core’ rays and emission from rays near the limb. Rotation also modifies line profiles of wind diagnostics such as He ii λ4686 and Hα and should not be neglected when inferring the actual stratification, level and nature of wind structures.