  We show that recent ARPES results on the "unusual" oxygen isotope shift in
the real part of the self-energy in the optimally doped $Bi2212$ samples can be
qualitatively (and semi-quantitatively) explained by the theory of the
electron-phonon interaction (EPI) elaborated few decades ago. However, for a
quantitative analysis of the ARPES spectra it is necessary to know the momentum
dependence of the EPI, the Coulomb contribution at high energies and the
background due to impurities and defects.
