  Massive star evolution remains only partly constrained. In particular, the
exact role of rotation has been questioned by puzzling properties of OB stars
in the Magellanic Clouds. Our goal is to study the relation between surface
chemical composition and rotational velocity, and to test predictions of
evolutionary models including rotation. We have performed a spectroscopic
analysis of a sample of fifteen Galactic O7-8 giant stars. This sample is
homogeneous in terms of mass, metallicity and evolutionary state. It is made of
stars with a wide range of projected rotational velocities. We show that the
sample stars are located on the second half of the main sequence, in a
relatively narrow mass range (25-40 Msun). Almost all stars with projected
rotational velocities above 100 km/s have N/C ratios about ten times the
initial value. Below 100 km/s a wide range of N/C values is observed. The
relation between N/C and surface gravity is well reproduced by various sets of
models. Some evolutionary models including rotation are also able to
consistently explain slowly rotating, highly enriched stars. This is due to
differential rotation which efficiently transports nucleosynthesis products and
allows the surface to rotate slower than the core. In addition, angular
momentum removal by winds amplifies surface braking on the main sequence.
Comparison of the surface composition of O7-8 giant stars with a sample of B
stars with initial masses about four times smaller reveal that chemical
enrichment scales with initial mass, as expected from theory. Although
evolutionary models that include rotation face difficulties in explaining the
chemical properties of O- and B-type stars at low metallicity, some of them can
consistently account for the properties of main-sequence Galactic O stars in
the mass range 25-40 Msun
