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 Mac App Store using the same Apple ID you used to purchase OALD8 originally. Then go to the ‘Catalogue of dictionaries’ within the newly updated OALD app. There you should see both OALD8 and OALD9. Click on the button below the OALD8 icon to re-download the content for free. The process will be different depending on when you first bought OALD8. You may be able to click ‘Download’ straight away. You may need to click ‘Buy’ but in this case you will not be charged for the purchase.