Context : Cygnus OB2 is a rich OB association in the Galaxy located at only \sim 1.4 kpc from us 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 . Aims : Our aim is to determine silicon and oxygen abundances for a sample of eight early-type slow rotators ( with rotational velocities below 80 km s ^ { -1 } ) 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 . Methods : We have performed a spectroscopic analysis of a sample of late O and early B stars with low rotational velocity in Cygnus OB2 , 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 ( effective temperature and surface gravity ) as well as the silicon and oxygen surface abundances . Results : 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 . Conclusions : 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 H II regions . We also find that the oxygen scatter within Cygnus OB2 is at least of the same order than among H II regions at similar Galactocentric distance .