In order to investigate oxidation-induced stacking faults (OSF), Cu-decorated Si wafers have been examined by dark-field optical microscope (OM), transmission X-ray topography (XRT), and scanning photoluminescence (sPL). It is found from OM observation that there are bright pits with several 10 µm sizes, forming a single ring, although there are many fine pits. On the other hand, a few subsidiary rings, in addition to the main OM ring, are observed by XRT and sPL measurements, although they do not directly correlate each other. It is found from microscopic PL measurement that there are dark spots, corresponding to the bright OM pits in the main OSF ring. By comparing the OM and sPL results, it is clearly found that the dark PL spots are corresponding not to fine OM pits but large OM pits with several 10 µm sizes, which seem to be due to copper oxides or precipitates on the surface. It may be emphasized that the sPL technique presented here may be used instead of OM to observe the OSF ring in Cu-decorated Si wafers, and also instead of XRT to observe the OSF ring in standard Si wafers annealed without copper decoration, because PL is more sensitive than OM observation and its intensity is related to the lifetime and concentration of carriers.