In high temperature cuprate superconductors like YBCO, there are intermediate copper-oxide planes, where $Cu$ and $O$ atoms are arranged alternatively in a square lattice. In this arrangement, the $Cu$ atoms generally make 4 in-plane and 2 axial bonds, and each $O$ makes 2 in-plane bonds (see attached image).
Oxygen's electron configuration $[He]2s^2 2p^4$, and can indeed form two bonds. However, copper has the electronic configuration $[Ar] 3d^{10} 4s^1$ which in the crystal field of the lattice becomes $[Ar] 4s^2 3d^9$, with the $3d_{x^2-y^2}$ orbital half-filled.
What hybridisation does the $Cu$ orbitals undergo to be able to form four in-plane bonds. To form 6 bonds, it seems that the five 3d and one 4s orbitals will hybridise, but the $3d_{x^2-y^2}$ orbital is well separated from the other 3d orbitals in energy.