Using a simple model based on the knowledge of spherical and deformed Woods-Saxon potentials, it is shown that the recent observation of halo phenomena in $^{29}$F can be interpreted as evidence for the prolate deformation of the ground state of $^{29}$F. The prolate deformation is the result of the shell structure, which is unique in one-neutron resonant levels, in particular near degeneracy of the neutron $1f_{7/2}$ and $2p_{3/2}$ resonant levels, together with the strong preference of prolate shape by the proton number $Z=9$. On the other hand, in oxygen isotopes spherical shape is so much favored by the proton number $Z=8$ that the presence of possible neutron shell-structure may not make the system deformed. Thus, the strong preference of particular shape by the proton numbers 8 and 9, respectively, together with a considerable amount of the energy difference between the neutron $1d_{3/2}$ and $2s_{1/2}$ orbits in oxygen isotopes seems to play an important role in the phenomena of oxygen neutron drip line anomaly, as was suggested by H. Sakurai et al. in 1999.