Playing with mid-to-high handicappers each week, one question that usually arises is 'keeping the left arm straight.' Here's a nice video by Jeff Evans discussing the straight left arm and how to keep it from being bent.
IMO, I pretty much agreen with everything Jeff says, although I think it should be noted that the left arm does not have to be straight in the backswing or at P4 and often times it's got a little bit of curve to it, and is not perfectly straight.
I think Strange is more of an extreme example of a bent left arm at P4, but that is something common of Jimmy Ballard's teaching philosophy. Although I think if want a more perfectly straight left arm, then I would suggest the Stack and Tilt method. I think the 'curved' left arms of O'Grady and Palmer are more than fine and Evans' tips can help you achieve that.