Download
App stores:
Install or Update Öffi from F-Droid (recommended for most Android devices)
Install or Update Öffi from Google Play
Install or Update Öffi from Amazon Appstore (for Kindle Fire and Fire Phone only)
Direct download:
Download/Install Öffi 10.5.1-aosp from here, if you cannot or do not want to use Google Play
Versions for old devices:
Download/Install Öffi 9.07.7-google from here, only in case you own an Android 4.0 device
Download/Install Öffi 8.46 from here, only in case you own an Android 2.2 device
Download/Install Öffi 7.99 from here, only in case you own an Android 2.1 device
Download/Install Öffi 6.88 from here, only in case you own an Android 1.6 device
Download/Install Öffi 5.24 from here, only in case you own an Android 1.5 device