We review the predictions relative to the evolution of the [O/Fe] as a func
tion of [Fe/H] in the Galaxy. In particular, we discuss the influence of di
fferent stellar yields as well as different SN (II, I) rates on the [O/Fe]
ratio. Possible explanations for the steep rise of the [O/Fe] ratio at low
metallicities, as indicated by recent observations, are critically discusse
d. (C) 2001 Elsevier Science B.V. All rights reserved.