You should be able to reinstall a previously purchased copy of OALD8 by going to Google Play > My apps & games > All. There you should see your previously purchased OALD8 app and be able to reinstall it.
If you cannot find and reinstall your app, the most likely reason is that you are not signed in to Google Play using the account that you used to buy the OALD8 app originally. You could try checking your emails to see if you can find the email receipt for your purchase and so confirm which account you used. You may also like to try contacting Google Play's support team for further advice.
Please see here for more information on reinstalling apps: https://support.google.com/googleplay/answer/113410