To control oxidation process precisely is important to fabricate small-aperture oxide-VCSELs with high reproducibility and uniformity. A real-time observation of the oxidation through the CCD camera was previously reported. However, it is difficult to observe the oxidation rate of a production-scale VCSEL wafer in a chamber by IR microscope. To measure simply and to control precisely the depth of AlAs-oxidation, the oxidation was in-situ monitored via optical probing technique for AlAs lateral oxidation (OPTALO). The substrates of the 780-nm oxide VCSELs were prepared with dry-etched 20-micron stripes filled over 7mm square area. The trenches of the stripes grooved periodically were 10 micron wide and 5 micron deep, which enables AlAs layer to be oxidized laterally. An optical fiber was placed over the substrate to illuminate using a halogen lamp and to collect the reflection to a spectrometer. Due to the spectral difference between the oxidized and non-oxidized area, the reflectivity linearly changes during the oxidation according with the ratio of the oxidation and non-oxidation area. The side-lobes of the spectra were averaged as the OPTALO signal, then the transients of the signals showed good agreement with transients of the depth of AlAs lateral oxidation.