Examples of model fits to four O-type main sequence stars. Areas strongly affected by nebular contamination are marked in grey. Spectral types based on He Iλ4921 and He IIλ5411 equivalent widths are also included (see Sect. 3.2). As expected, the He Iλ4921 line increases as we move towards later spectral types, while the He IIλ5411 decreases, thus supporting our algorithm for spectral classification.