The effects of surface diffuseness on the single particle level structure and wave functions are investigated in a pure single particle model. It is shown that the ground state and the first excited state of O17, which are interpreted as giving 1d5/2 and 2s1/2 shell levels respectively, and low energy elastic scattering of neutrons by O16 can be well reproduced by a diffuse potential well. In particular, it is pointed out that the relative location of 2s1/2 and 1d5/2 levels depends sensitively on finer details of the surface diffuseness. The modifications of the single particle wave functions from pure harmonic oscillator functions are further investigated, and they are found to be small up to 1p states, but for higher states they may be so large that with pure oscillator functions one cannot hope to make quantitative discussions when treating near A =16 nuclei.