Using high resolution optical spectroscopy (R˜40,000-50,000) we have derived the Li and Zr abundances of a large sample of galactic O-rich AGB stars. Our chemical analysis shows that some stars are Li overabundant while others are not. The observed Li overabundances are attributed to the activation of the so-called ``hot bottom burning'' (HBB) process, confirming that they are actually massive AGB stars. However, these stars do not show the zirconium enhancement (taken as a representative for the s-process element enrichment) usually associated to the third dredge-up. Our study reveals that the s-process element abundances of the more massive O-rich AGB stars in our Galaxy are dramatically different from those found in the equivalent population of AGB stars in the Magellanic Clouds. We conclude that probably the different metallicity environment can explain the differences observed.