Once you have paid for OALD9, you should not be required to pay again to re-download it to your device or to install it on another of your devices. Firstly, please ensure that you are signed in to the App Store with the same Apple ID you used to make the purchase. Then, in the Catalogue tab of the OALD app, tap Restore. This should allow you to access in-app purchases made with this Apple ID, i.e. to download OALD9 again at no further cost.
If this does not work, then please delete the OALD app and restart or 'force restart' your device. Once your device has restarted, please reinstall the OALD app from the App Store (making sure that you are signed in with the same Apple ID you used to purchase OALD9 originally), then tap the Restore button in the Catalogue tab.
You could also try signing out of your App Store account (Apple ID), then reopening the OALD app and pressing the Restore button again – you will then be prompted to sign in with your Apple ID while within the app and sometimes this can make a difference.
If you are having trouble restoring OALD8, please see here: https://oxfordeltdictionaries.zendesk.com/hc/en-us/articles/114094172252-Why-can-t-I-see-OALD8-in-the-Catalogue-tab-.
If you continue to experience difficulties, please contact us via the email link in the app.