If you had previously bought the OALD8 app, then yes, you can have both OALD8 and OALD9 installed simultaneously in the updated OALD app. To do this, go to the Catalogue tab in the app. There you can tap Get to re-download OALD8 for free and make the in-app purchase to upgrade to OALD9. Once both dictionaries are downloaded, you can switch between the two from the Catalogue tab.
If you had not previously bought the OALD8 app, then you will only be able to install OALD9 now.