Octahedral complexes:

In an octahedral field those d orbital whose lobes are directed along the axis will experience greater interaction from the point charges placed in those directions. These are the d_{x² −y²} and d_z² orbitals- the electron of these orbital will experience stronger repulsion from the point charges in the environment i.e. these orbitals will be destabilized & their energy will be raised to some extent.

But the remaining three orbital d_xy, d_xz and d_yz have their lobes between the axes (45^°) and will be less effected by the point charges in the environment i.e. these orbitals will be subject less electronic repulsion and to maintain the barycentre or center of gravity, energy of these orbitals will be lowered some extent.