The semiquantitative analysis of optical absorption for interstitial oxygen in silicon was carried out using the molecular orbital theory and compared with results from Fourier transform‐infrared equipment having a high resolution of 0.05 cm−1. Six finely split peaks were observed with the wave numbers of 1120.2, 1123.6, 1128.3, 1132.8, 1133.5, and 1136.4 cm−1at 30 K, among which 1120.2, 1132.8, and 1133.5 cm−1were newly observed. It is concluded that there seems to be a reliable correlation between the observed band splitting and the likely energy transitions from anS6symmetry model. Fine splitting of the absorption peaks at low temperature indicates the close relationship between the local Si—O—Si bond and six nearest neighbor silicon atoms formingS6symmetry. Absorption peaks also were narrower and higher as the measurement temperature was lowered. Hence, it can be said that low‐temperature measurement improves the oxygen detectability by a factor of 10 compared with measurement at room temperature.