O'Brien is leaving off-camera, so technically it's not a spoiler. You'll tune in to Season 4 and she'll already be gone.
I have no problem at all with Dan wanting to leave. And some people aren't looking for fame and fortune, they just want a little variety, so I say more power to him. I'm fine with Matthew dying too, it just would have been nice if it hadn't happened THE EXACT SAME DAY his son was born.