If you had bought the OALD8 app previously, then you should be able to see OALD8 in the Catalogue tab so that you can re-download it and continue using it for free. If you cannot see OALD8 in the Catalogue tab, 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 the Restore button in the Catalogue tab. You should then see both OALD8 and OALD9. Tap on Get to reinstall OALD8 at no cost.
If you are still unable to see OALD8 in the Catalogue tab, then it suggests that you were using Family Sharing to access OALD8 previously. You will no longer be able to do this. For more information on why Family Sharing doesn't work anymore, please see here: https://oxfordeltdictionaries.zendesk.com/entries/108385196-Why-doesn-t-Family-Sharing-work-anymore-
If you had not previously purchased the OALD8 app, then you will only be able to install OALD9 now, i.e. you will only see OALD9 in the Catalogue tab.