This is an Open Source & Unofficial app which just uses REST API of OneSignal made for Developers and App owners.

Source code: https://github.com/sdzshn3/OneSignalAPI

This app has lot of customization features of notification like including segments, exclude segments, send using filters, send to particular platform, see notification preview, add image, add additional data, action buttons, lots of platform settings, see notification history, check stats of sent notification, and you can also schedule your notification.