The influence of apex oxygens on the pairing interaction between the holes in Cu-O clusters has been investigated by using an exact diagonalisation method. A model Hamiltonian consisting of on-site and nearest-neighbour Coulomb repulsion terms is defined for 9-site planar and 11-site Cu-O clusters having two additional O atoms on the top of Cu-sites. The results indicate that these apex oxygens may play a significant role in enhancing pairing in two-dimensional Cu-O planes.