Antiy-AVL detected Trojan/Win32.Possiblethreat
Bkav Pro detected W32.AIDetectMalware
TrendMicro-HouseCall detected TROJ_GEN.R002V01K523
Trapmine detected Malicious.moderate.ml.score
Heuristic detection. They're guessing based on behavior and machine learning etc. Probably flagged the Opera installer because it's the net installer that fetches Opera's program files from Opera's servers. They probably wouldn't have a problem with Opera's offline package that includes the program files.
Opera's installer is also compressed with UPX to make it smaller. Some engines consider this malicious even when it's not.
Those detections all look like false positives to me.