Opus Dei seems to have a wide range of practices, from the unremarkable to the totally bizarre. There are credible reports of Opus Dei members and groups engaging in things like self-flagellation to the point of major bleeding. Then again, I don't think anyone believes that Opus Dei member Antonin Scalia engages in such practices. The public face of Opus Dei disclaims any involvement in such things; the hard-core conspiracy theorists contend that the palatable public face is just to lure people into the "cult", and that the intention is to gradually pull as many people as possible into the extreme practices. The truth is probably that the whole spectrum exists within the many groups operating under the Opus Dei label, and that the extremists conceal their activities more to avoid having the Vatican publicly disavow their claim to be officially part of Opus Dei, than to deceive new members about the true nature of the organization.
There are credible reports of Catholics (Paul VI) wearing hair shirts and in Doctors of the Church (St. Theresa of Avila) engaging in self-flagellation. There's nothing new or secret to such acts of mortification. If anyone is doing serious harm to himself, it's most assuredly without the approval of the Church or the leadership of Opus Dei.
I'd just love to see these 'credible sources'. There may be some possibility that in a backwards culture someplace in the world, (where culture is mixed in with relgion), that something like this could occur. But I think it's just more anti-Catholic b.s.