  The 2--point functions for $\Delta S=1$ current-current and QCD-penguin
operators, as well as for the $\ds$ operator, are calculated at the
next-to-leading order. The calculation is performed in two different
renormalization schemes for $\gamma_5$, and the compatibility of the results
obtained in the two schemes is verified. The scale- and scheme-invariant
combinations of spectral 2--point functions and corresponding
Wilson-coefficients are constructed and analyzed. For $\Delta S=1$, the QCD
corrections to the CP-conserving part, dominated by current-current operators,
are 40\%--120\% at $q^2=\left(1-3\,\mbox{\rm GeV}\right)^2$, whereas the
correction to the imaginary part, mainly coming from the penguin operator
$Q_6$, are 100\%--240\%. The large size of the gluonic corrections to
current-current operators provides a qualitative understanding of the observed
enhancement of $\Delta I=1/2$ transitions. In the $\Delta S=2$ sector the QCD
corrections are quite moderate ($\approx -20\%$).
