  Massive stars are among the main cosmic engines driving the evolution of
star-forming galaxies. Their powerful ionising radiation and stellar winds
inject a large amount of energy in the interstellar medium. Furthermore,
mass-loss ($\dot{M}$) through radiatively driven winds plays a key role in the
evolution of massive stars. Even so, the wind mass-loss prescriptions used in
stellar evolution models, population synthesis, and stellar feedback models
often disagree with mass-loss rates empirically measured from the UV spectra of
low metallicity massive stars. The most massive young star cluster in the low
metallicity Small Magellanic Cloud galaxy is NGC346. This cluster contains more
than half of all O stars discovered in this galaxy so far. A similar age,
metallicity ($Z$), and extinction, the O stars in the NGC346 cluster are
uniquely suited for a comparative study of stellar winds in O stars of
different subtypes. We aim to use a sample of O stars within NGC346 to study
stellar winds at low metallicity. We mapped the central 1" of NGC346 with the
long-slit UV observations performed by the Space Telescope Imaging Spectrograph
(STIS) on board of the {\em Hubble Space Telescope} and complemented these new
datasets with archival observations. Multi-epoch observations allowed for the
detection of wind variability. The UV dataset was supplemented by optical
spectroscopy and photometry. The resulting spectra were analysed using a
non-local thermal equilibrium model atmosphere code (PoWR) to determine wind
parameters and ionising fluxes. The effective mapping technique allowed us to
obtain a mosaic of almost the full extent of the cluster and resolve stars in
its core. Among hundreds of extracted stellar spectra, 21 belong to O stars.
Nine of them are classified as O stars for the first time. We analyse, in
detail, the UV spectra of 19 O stars... (continued)
