Hello Brixton_Hill,
When did you purchase the app? Have you restarted the iPhone since you installed this app? If not, Restart your iPhone - Apple Support. Often this simple step will refresh the apps and software on the device to solve numerous problems you may encounter.
Open the App Store and check for app updates. If you see an available update for the app, complete the update. If there is no available update, delete the app and reinstall it on your iPhone. You can follow these steps to Delete apps on your iPhone, iPad, and iPod touch - Apple Support. These steps will help to make sure there are no issues with the currently installed app software.
If you have followed these steps and still have an issue, you may want to reach out to the Oxford English Dictionary (OED) app developer directly. For more information on how to contact them, check out this steps on this page: How to contact an app developer - Apple Support.
We hope this information helps you out.
Kind regards!