We study the multi-transverse mode distributions and the wavelength splittings with different designed oxide apertures of the oxide-confined VCSEL. By developing the effective index model and BPM algorithm theory, the characteristics of transverse optical field distribution are calculated with circular aperture and ellipsoid aperture, which are compared with our experimental results of multi-wavelength spectra of high-order transverse modes. The results show that the orthogonality of the different crystal orientation modes will be broken by the oxidation-induced ellipsoid aperture, and the maximum wavelength spltting of the degenerated high-order mode is 0.037 nm, which can be reduced as the diameter of aperture increases. The results in this paper will provide a useful reference for multi-transverse mode locking of oxide-confined VCSELs.