In pulsed laser deposition (PLD) of oxides, usually high oxygen pressures are used to incorporate oxygen in the as-grown film at high substrate temperature and to prevent the growing film to be bombarded by high-energetic particles, originating from the plasma. Furthermore, the background pressure influences the shape and size of the plasma and, therefore, the deposition rate and homogeneity of the film and the energy of the ad-atoms on arrival. Reflection high energy electron diffraction (RHEED) is often used to monitor the growth of oxide thin films. In general, low background oxygen pressures are used. Because of the low oxidation power at low pressures, low substrate temperatures have to be used. This hampers, in general, the film crystallinity. We have developed a PLD-RHEED system for growth monitoring during the deposition of oxides at standard PLD conditions. Using a two-stage differential pumping system, clear diffraction patterns are observable up to 50 Pa. Therefore, monitoring and control of thin film growth by PLD on an atomic level is feasible, even in quite high background pressures. The influence of the substrate treatment, e.g. annealing and etching, on the growth of atomic layers of SrO and BaO, as well as the homo-epitaxial growth of SrTiO3, will be discussed in this contribution.