Opera recently changed their User Agent string in Opera 15 and AWStats incorrectly detects this browser as Chrome. A typical user agent string for Opera 15(+) is:
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_8_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/28.0.1500.52 Safari/537.36 OPR/15.0.1147.132
Fix see https://sourceforge.net/p/awstats/discussion/43428/thread/28ee36de/#e663