Hmmm... what kept them from freezing before? If they were going to freeze, they'd have done it while using oil. All you need is an hour or so with no call for heat and they'd catch. I'm guessing if they haven't frozen by now, they must be close enough to the ceiling or something. It's one of those "we'd have to see it" circumstances. "Pipes in the attic" can mean anything from them being right out in the open to against the sheet rock and under 12" of insulation.