There is no obligation to pay to continue using the OALD app - you can re-download and keep using OALD8 for free.
To do this, first please make sure that you are signed in to the App Store using the same Apple ID you used to purchase OALD8 originally. Then tap on the Catalogue tab within the newly updated OALD app. There you should see both OALD8 and OALD9 (you may need to tap Restore first). Tap on Get to reinstall OALD8 at no cost.
If you have any problems while trying to re-download your OALD8, then these should be solved by updating your device to iOS 9.2 or later.