PushExit guides
What you can do with the push subscribers you already have, by hand, with nothing but curl and a few lines of Python.
How to keep sending push to your OneSignal subscribers through your own Firebase
Over OneSignal’s 1,000 MAU free limit? Export your subscribers’ FCM tokens through the OneSignal API and send to them directly with Firebase Cloud Messaging v1. No new app version, tested on Android.