  Our goal is to determine the stellar and wind properties of seven O stars in
the cluster NGC2244 and three O stars in the OB association MonOB2. These
properties give us insight into the mass loss rates of O stars, allow us to
check the validity of rotational mixing in massive stars, and to better
understand the effects of the ionizing flux and wind mechanical energy release
on the surrounding interstellar medium and its influence on triggered star
formation. We collect optical and UV spectra of the target stars which are
analyzed by means of atmosphere models computed with the code CMFGEN. The
spectra of binary stars are disentangled and the components are studied
separately. All stars have an evolutionary age less than 5 million years, with
the most massive stars being among the youngest. Nitrogen surface abundances
show no clear relation with projected rotational velocities. Binaries and
single stars show the same range of enrichment. This is attributed to the youth
and/or wide separation of the binary systems in which the components have not
(yet) experienced strong interaction. A clear trend of larger enrichment in
higher luminosity objects is observed, consistent with what evolutionary models
with rotation predict for a population of O stars at a given age. We confirm
the weakness of winds in late O dwarfs. In general, mass loss rates derived
from UV lines are lower than mass loss rates obtained from Ha. The UV mass loss
rates are even lower than the single line driving limit in the latest type
dwarfs. These issues are discussed in the context of the structure of massive
stars winds. The evolutionary and spectroscopic masses are in agreement above
25 Msun but the uncertainties are large. Below this threshold, the few
late-type O stars studied here indicate that the mass discrepancy still seems
to hold.
