  We present the first installment of a massive spectroscopic survey of
Galactic O stars, based on new, high signal-to-noise ratio, R~2500 digital
observations from both hemispheres selected from the Galactic O-Star Catalog of
Ma\'iz Apell\'aniz et al. (2004) and Sota et al. (2008). The spectral
classification system is rediscussed and a new atlas is presented, which
supersedes previous versions. Extensive sequences of exceptional objects are
given, including types Ofc, ON/OC, Onfp, Of?p, Oe, and double-lined
spectroscopic binaries. The remaining normal spectra bring this first sample to
184 stars, which is close to complete to B=8 and north of delta = -20 and
includes all of the northern objects in Ma\'iz Apell\'aniz et al. (2004) that
are still classified as O stars. The systematic and random accuracies of these
classifications are substantially higher than previously attainable, because of
the quality, quantity, and homogeneity of the data and analysis procedures.
These results will enhance subsequent investigations in Galactic astronomy and
stellar astrophysics. In the future we will publish the rest of the survey,
beginning with a second paper that will include most of the southern stars in
Ma\'iz Apell\'aniz et al. (2004).
