Download
App stores:
Install or Update Öffi from F-Droid (recommended for most Android devices)
Install or Update Öffi from Google Play
Direct download:
Download/Install Öffi 12.1.18-aosp from here, if you cannot or do not want to use the services listed above
Versions for old devices:
Warning: these old versions are unsupported and will not receive updates any more.
Download/Install Öffi 11.4.5-aosp from here, only in case you own an Android 4.4 device
Download/Install Öffi 10.6-aosp from here, only in case you own an Android 4.1, 4.2 or 4.3 device