@AOKP OK, I've had a first crack at a nodebb-plugin-onesignal notifier. You can give it a try here.
Ignore the README, it was forked and modified from the pushbullet plugin and I haven't got around to updating the README and publishing on npm yet.
You need to populate your OneSignal app_id and API key within the admin section. You can get this from the OneSignal website under your applications
Keys & Id section
I haven't done the Android integration yet, but you can get your player_ids from the OneSignal site here
I have exposed a REST API for adding player id's at
/api/me/onesignal/devices. An authenticated
POST with either a valid cookie or bearer token from the nodebb-plugin-write-api will work with the following JSON body
"player_id" : "player-id-here"
Once this has been completed, notifications should work which you can test by going to the OneSignal settings within your profile.
Appreciate any feedback