While Ohlendorf had a couple of serviceable seasons in the Pirates' rotation in 2009 and 2010, his 2011 performance is the kind that can only be called "pitching" with the loosest of definitions. Did he throw a baseball towards the plate from the mound? Yes. Did he do it better than John Lackey? Not appreciably. Enough said.
But just in case that doesn't do it for you, here are the numbers: in the 38 innings he was given by the Pirates, Ohlendorf pitched to an 8.15 ERA thanks largely in part to a 36.2% GB rate coupled with a 15.8% HR/FB. He was quickly let go by the Pirates, and was out of baseball until the Red Sox came calling with a minor league contract.
In a way Ohlendorf makes sense, if only because he's a minor league signing and it's almost impossible for depth at a negligible cost to be bad. On the other hand, it's hard to imagine that Ohlendorf will ever make his way into the rotation picture, or even come close. Even with his peripherals having stayed at their usual (mediocre) levels, Ohlendorf is just about the worst possible fit imaginable for Fenway given his fly ball rate. Add a transition to the AL East, and he's probably behind even the likes of Carlos Silva.