In Android oreo Background Execution Limits are introduced. there are certain limits in background running operation in android. if app is in idle mode or in foreground then this limitation is not applied. its apply when app is in background. Also implicities broadcast will not work. you have to create explicite broadcasts.
There is also background service limitations, service will not work in background in android O. You can replace background services with JobScheduler jobs. Android O offers many improvements to JobScheduler that will make it easier to replace services and broadcast receivers with scheduled jobs; for more information, see JobScheduler improvements here.
if you have integrated firebase messaging push notification then,
Add new/update firebase messaging dependencies for android O (Android 8.0), due to Background Execution Limits.
upgrade google play services and google repositories if needed.