We present the results of the spectral analysis of 11 OB stars in Cyg OB2, among them seven giants and supergiants. The projected rotational velocities of these stars are low or moderate. We find that only one of the stars (previously classified as luminosity class V) shows helium enhancement, and that this turns out to be compatible with standard evolutionary models without rotation. Only a second object gives an abundance that could be larger than solar. In addition, these two stars are found to be the most luminous in the sample. In summary, no helium discrepancy is found for the stars analysed in Cyg OB2. The cause of this result is speculated to be due to: a) the youth of the stars studied, b) the low rotational velocity of the sample observed, or c) a combination of both. The massive stars in Cyg OB2 are found to have ages between 1 and 5 Myr, and the most massive ones have initial evolutionary masses in excess of 100 M/Msunsun. Thus we confirm that Cyg OB2 is a young association rich in very massive stars. We study a possible correlation between the helium abundance and the stellar rotation and conclude that present data are consistent with the hypothesis that mixing processes probably related to rotation are present in the massive stars and in some cases strongly influence their early evolutionary phases. The Isaac Newton Telescope is operated on the island of La Palma by the RGO in the Spanish Obervatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.