Differences between incoming observations and a short-term model forecast, the observed-minus-forecast residuals, are computed as a part of the statistical analysis scheme in the assimilation process. Their global and regional statistics are routinely evaluated. Changes in these statistics were seen and they corresponded to changes in instrument performance and calibration. We show examples of such changes for NOAA-14 SBUV/2 and Earth Probe TOMS instruments. Similar monitoring will be used for new ozone observations from NASA's EOS Aura satellite, from the AIRS instrument on EOS Aqua and from instruments on European Space Agency's ENVISAT.