You could trade them on Points.com's "GPX" for points in another program. I haven't used it and it looks sort of cumbersome (and if I read the chart right, there'd be a fee of $10 per thousand miles), but OnePass does belong to this part of Points.com and as long as there's another GPX partner you want miles in, it's better than nothing. Admittedly, the list of other GPX partners is slim pickings...

Or you really don't expect to collect any more OnePass miles anytime soon, you could just make a charitable contribution - CO has lots of options on their website.