C's are pretty much letting OKC shoot open midrange jumpers, and on offense Boston's got no flow, no oomph. One of the announcers might've hit it on the head when he said: "No Durant, no Green, home game for Boston, they probably figured it was a gimme." And that's what makes watching Boston regular-season basketball so much FUN. 
I can't really fault the offense. They're moving the ball well, and are shooting something like 55%.
The defense has been lazy at times, but OKC also has been extremely hot shooting. I think on the average night, even with this level of effort, the Celtics would be up by 10+. However, when OKC doesn't miss on its mid-range jumpers, it's hard to build a lead.