If you are receiving a pop-up along the lines of 'In-app purchases are not allowed', this is most likely because you have disabled in-app purchases in your device's settings. To enable in-app purchases (so to be able to re-download OALD8 and to buy OALD9), please go to Settings > General > Restrictions and scroll down to In-App Purchases. Enable this option by toggling the switch to green. More information is available here: https://support.apple.com/en-gb/HT204396
You should then have no problem downloading the dictionaries from the Catalogue tab, provided your device has been updated to iOS 9.2 or later.