Hi Chris. I haven't bent Oak before but I've bent Cedar, Mahogany and Walnut. I can't really tell from the pictures but it may just be that the ribs are sawn on the wrong grain axis. You want the growth rings to be parallel on the wide side of the workpiece and cathedral shaped/coencentric on the narrow side (aka quarter sawn). You may have inadvertantly flat sawn your ribs.

If that's really what's happening then you will get less runout/less breakage if you change the grain orientation by sawing the wood the other way. I can't tell if this makes sense but I gotta go; the kids are screaming.

Good luck with it. You'll get it eventually.
www.ChipSandresky.com