I think you gotta take a good hard look at the over 8 in this one. Oswalt has not quite been himself all year, and Colorado has some solid bats in their lineup. They'll likely get to him for a few, and threaten the pen if it comes in.
And on the other side, Cook has proven to be very hittable this year. Even if Oswalt is sharp and returns to his old form, Houston could easily come close to 8 all by themselves against Cook and the back of the Colorado pen.
Having said that, I like Colorado at this line of about +150. But the over is the play I really like.