Oscar wasn't during a 5 defense reign. Sturm did defend against Murray and Macklin before GGG or Martinez, and beat Zbik and Sylvester too. Not that bad.
He doesn't count. This is for guys who defended belts five times. But Broner has fought good oppositio;, he just lost to his best opponents.
Not that Sven was a WBO fighter but during the 90s he seemed to do a pretty crap carousel with Calazaghe as to people Warren could afford to get over.