  We investigate the mass spectra of open heavy flavor mesons in an external
constant magnetic field within QCD sum rules. Spectral ans\"atze on the
phenomenological side are proposed in order to properly take into account
mixing effects between the pseudoscalar and vector channels, and the Landau
levels of charged mesons. The operator product expansion is implemented up to
dimension-5 operators. As a result, we find for neutral D mesons a significant
positive mass shift that goes beyond simple mixing effects. In contrast,
charged D mesons are further subject to Landau level effects, which together
with the mixing effects almost completely saturate the mass shifts obtained in
our sum rule analysis.
