  We quantitatively investigate the extent of wind absorption signatures in the
X-ray grating spectra of all non-magnetic, effectively single O stars in the
Chandra archive via line profile fitting. Under the usual assumption of a
spherically symmetric wind with embedded shocks, we confirm previous claims
that some objects show little or no wind absorption. However, many other
objects do show asymmetric and blue shifted line profiles, indicative of wind
absorption. For these stars, we are able to derive wind mass-loss rates from
the ensemble of line profiles, and find values lower by an average factor of 3
than those predicted by current theoretical models, and consistent with H-alpha
if clumping factors of f_cl ~ 20 are assumed. The same profile fitting
indicates an onset radius of X-rays typically at r ~ 1.5 R_star, and terminal
velocities for the X-ray emitting wind component that are consistent with that
of the bulk wind. We explore the likelihood that the stars in the sample that
do not show significant wind absorption signatures in their line profiles have
at least some X-ray emission that arises from colliding wind shocks with a
close binary companion. The one clear exception is zeta Oph, a weak-wind star
that appears to simply have a very low mass-loss rate. We also reanalyse the
results from the canonical O supergiant zeta Pup, using a solar-metallicity
wind opacity model and find Mdot = 1.8 \times 10^{-6} M_sun/yr, consistent with
recent multi-wavelength determinations.
