A few step are necessary to make sure that Life Reminders works without any issue on OPPO phones, here they are:
- Go to settings --> advanced settings --> battery manager --> protected apps, then find Life Reminders, and "protect" it.
- Go to Security Center-->Privacy permission-->Startup manager, then find Life Reminders then make sure that is is allowed
- Go to settings --> apps --> advanced --> ignore battery optimizations, then find Life Reminders and ignore it.
- Go to settings --> notification panel & status bar --> notification center, then find Life Reminders, then activate "allow notifications" and also "priority display". You have to activate the priority part too to make sure you get notifications.
- Open the Life Reminders hit the square (aka 'app overview" or Recent apps) button on the navigation bar, then pull down on that app to lock it into place. From here on out, even if you're "clearing all apps," the ones you've locked will remain.
Make sure that you restart your device after this.
PS: These infos where taken from the following page: https://www.forbes.com/sites/bensin/2017/07/28/how-to-fix-push-notifications-on-oppo-phones/#7c627a391735