The Ogio bag does have tie downs
There are 4 straps that are looped at one end with the male half of a banana clip on the other (adjustable at the end with the clip). There is a female banana clip on each lower corner of the bag.
Just wrap each strap around the point on the bike you want to attach it to, pass it through its own loop, click to the bag and cinch it tight. I use the rear rack for the aft straps and the passenger grab rails for the forward ones. Since the end that attaches to the bike is just pulled through its own loop, you don't have to worry about excess strap flapping around and beating up that fragile finish.
Once you're happy with where you anchor each strap, the bag goes on and off quickly.
I like the outside pockets for the "quick grab" stuff - change, sunglasses, wallet, etc. the bigger compartment is perfect for a change of clothes, raingear and spare face shield (I wear a smoked/tinted one in bright sun).