The OALD9 app appears to be free but when I download it, I am asked to make an in-app purchase. Why is this?

The OALD9 app is free to download and gives you 100 sample entries from the dictionary so that you can try out what the app has to offer. However, you will not be able to cross-refer from these sample entries to other words that are in the full dictionary. To have access to the full dictionary you will need to buy it via in-app purchase.

We make this very clear in the app’s description in Google Play.