  Cygnus OB2 is a rich OB association in the Galaxy which has experienced
intense star formation in the last 20-25 Myr. Its stellar population shows a
correlation between age and Galactic longitude. Exploring the chemical
composition of its stellar content we will be able to check the degree of
homogeneity of the natal molecular cloud and possible effects of
self-enrichment processes. Our aim is to determine silicon and oxygen
abundances for a sample of eight early-type slow rotators in Cygnus OB2 in
order to check possible inhomogeneities across the whole association and
whether there exists a correlation of chemical composition with Galactic
longitude. We have performed a spectroscopic analysis of a sample of late O and
early B stars with low rotational velocity, which have been chosen so as to
cover the whole association area. We have carried out an analysis based on
equivalent widths of metal lines, the wings of the H Balmer lines and FASTWIND
stellar atmosphere models to determine their stellar fundamental parameters as
well as the silicon and oxygen surface abundances. We derive a rather
homogeneous distribution of silicon and oxygen abundances across the region,
with average values of 12+log(Si/H)=7.53$\pm$0.08 dex and
12+log(O/H)=8.65$\pm$0.12 dex. We find a homogeneous chemical composition in
Cygnus OB2 with no clear evidence for significant chemical self-enrichment,
despite indications of strong stellar winds and possible supernovae during the
history of the region. Comparison with different scenarios of chemical
enrichment by stellar winds and supernovae point to star forming efficiencies
not significantly above 10%. The degree of homogeneity that we find is
consistent with the observed Milky Way oxygen gradient based on HII regions. We
also find that the oxygen scatter within Cygnus OB2 is at least of the same
order than among HII regions at similar Galactocentric distance.
