  The development of powerful infrared observational technics enables the study
of very extincted objects and young embedded star forming regions. This is
especially interesting in the context of massive stars which form and spend a
non negligible fraction of their life still enshrouded in their parental
molecular cloud. Spectrophotometric calibrations are thus necessary to
constrain the physical properties of heavily extincted objects. Here, we derive
UBVJHK magnitudes and bolometric corrections from a grid of atmosphere models
for O stars. Bessel passbands are used. Bolometric corrections (BC) are derived
as a function of Teff and are subsequently used to derive BC - spectral type
(ST) and Absolute Magnitudes - ST relations. Infrared magnitudes and, for the
first time, bolometric corrections are given for the full range of spectral
types and luminosity classes. Infrared colors are essentially constant.
Intrinsic H-K colors are 0.05 mag bluer than previously proposed. Optical
calibrations are also provided and are similar to previous work, except for
(B-V)0 which is found to be at minimum -0.28 for standard O stars, slightly
larger (0.04 mag) than commonly accepted.
