While neither backup is all that good, either of them would be a viable fantasy QB in this offense. The passing yards will be there. Look at O'Sullivan's numbers, he threw for 148 yards, a TD and 2 picks in about a half. I'll take 300 passing yards and 2 TD's over a game any time. He may throw more picks, but any QB in this offense will pile up some passing yards.
From what I am hearing around here, Martz thinks O'Sullivan makes better decisions than Orlovsky right now. It did not help that Orlovsky was hurt. For now, O'Sullivan is the backup. I can't say with any certainty how it will be in a month, or 2 months, but my personal opinion is it will stay like it is right now.