Changing the default iOS dictionary from New Oxford American Dictionary(NOAD) to Oxford English Dictionary(OED)?
It looks like this is possible in iOS 7. When you click to define a word, in the bottom left there is a "Manage" button. Here, you can tap the X for a dictionary that you want to delete from your device and tap the cloud button to download a dictionary you want. So in your case, tap the X to delete the NOAD from your iPhone, and tap the cloud to download the OED to your iPhone.