In this study, the ion yields of homonuclear diatomic molecules (O₂) were experimentally measured as a function of the laser intensity at two different wavelengths (800 and 1500 nm) in the linearly polarized (LP) and circularly polarized (CP) strong laser fields, exhibiting a peculiar dependence on wavelength and polarization. The O2+ yields were reduced at long wavelengths, and the yield difference between 800 and 1500 nm was larger in the CP field when compared with that in the LP field. The experimental results were qualitatively reproduced using the S-matrix model. Furthermore, the calculated electron-momentum distributions confirm the effect of the molecular orbitals on the strong-field single ionization of O₂.