Abstract: The subject of molecular orbital tomography is of very broad scientific interest. The question of whether it is possible to follow chemical transformations in time using the effect of high-order harmonic generation (HHG) is of primary practical importance. Moreover, investigations of the foundations of molecular orbital tomography may lead to valuable deep insights into the mechanism of atomic and molecular HHG. We investigate the orientation dependence of molecular HHG both numerically and analytically, and show that the molecular recollision electronic wave packets (REWPs) in HHG are closely related to the ionization potential as well as the particular orbital from which it ionized. As a result, in some energy regions the spectral amplitude of the molecular REWP can be significantly different from its reference atom (i.e., that with the same ionization potential as the molecule under study) due to interference between the atomic cores of the molecules. This finding is important for molecular orbital tomography using HHG.