We have studied the Cu-O vibrations in the MBa2Cu3O7 systems with M=Y,Eu,andGd by Raman spectroscopy. The observed Raman features are similar in the three systems. The four Raman bands in the 300-700 cm-1 frequency region exhibit isotopic shifts upon O18 substitution implying that these modes involve oxygen vibrations. In oxygen-deficient YBa2Cu3O7- samples prepared in the range 0< <0.7 within the orthorhombic structure, the A1g axial stretch mode is found to undergo both a softening and broadening with increasing oxygen deficiency. Furthermore, the Raman feature in the 550-600 cm-1 region shows an increase in intensity. The latter is attributed to disorder activation of the Cu-O chain mode, normally forbidden in the stoichiometric compound. These features and the emergence of a distinct Raman peak near 390 cm-1 in oxygen-deficient samples suggest that Raman spectroscopy could be used for characterizing oxygen stoichiometry of high-Tc superconductors.