Spectrograms with dispersions of 16 A mm ' have been used to determine the spectral classification of 130 0-type stars. We are able to introduce intermediate MK types, e.g., 07.5, by measures of the He I and He ii line strengths. We introduce a luminosity classification for the later 0-type stars which involves measures of the ratio of Si iv to (singlet) He I. About fifty 0-type stars have absolute magnitudes obtainable from membership in clusters and associations which have "well established" distances. These stars enable us to calibrate the luminosity criteria in terms of M,,. We define Of stars as those in which the N iii multiplet at XX4634, 4640,4641 is in emission above the continuum. Using only those 0 stars with known distances, we then find that the Of stars are exclusively in the hotter and brighter part of the H-R diagram. Although all 06 stars and earlier are Of, only the brighter later type 0 stars show these emission lines. Calibration of M with the spectral type and Of characteristics is provided for the 0 stars. The presence of these lines in emission is suggested to be due to extended, possibly expanding, envelopes around these luminous stars. The He ii 4686 line also appears in emission in some Of stars; there is a general tendency for the strength of this emission to be dependent on luminosity and effective temperature.