Collision processes between oxygen molecules and laser ablated fragments from YBa2Cu3Oy superconductor target are described. The space/time resolved photo-emission measurements of Cu and CuO were carried out in order to investigate the oxidation of the laser ablated fragments in O2 atmosphere. Increase of the photo-emission yield of CuO was observed over 100 mTorr and it showed maximum at 2 Torr. Velocity of CuO decreases abruptly at 200 mTorr. The velocity distribution spreads above 500 mTorr. An optimum range of oxygen pressure to fabricate superconducting thin films of Tc = 90 K was from 250 mTorr to 500 mTorr. Process of the reactive collisions of ablated fragments with O2 molecules in this range is discussed.