Of course O will revert to CM93 at some stage unless Raster charts
covers the whole area. Exactly when, at which scale this happens, can be adjusted with the Options->Display->Advanced ->Chart Zoom/Scale Weighting ->Raster.
I find that with the slider in default (=0) position the change happens when the scale gets smaller than the scale of the raster-chart....approximately. Increasing the slider value postpones when the change happens. For an example with slider set to +5, see below.