In the list of User-Agent strings there is very old Opera User-Agent. It is valid for old Presto based browser version <=12.
All modern Opera browser versions 15+ (including v. 49 last stable version) should be present there instead or as an additional item.
The example of Opera (Chromium based browser) User-Agent string:
Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.94 Safari/537.36 OPR/49.0.2725.64