In addition to the OneSignal web portal you may use OneSignal's Server REST API to send transactional push notifications programmatically.

OneSignal's REST API is documented at https://documentation.onesignal.com/reference

When sending notifications through OneSignal's API, you may target users by specifying predefined segments, filtering by parameters, or specific OneSignal IDs or other identifiers.

Send to Users

Sending notifications to all devices of a user is the most common use case when sending transactional or targeted notifications. You can easily do that using OneSignal ID or External ID. See User Management for details on configuring each of these strategies

Programmatically send notification to specific users

Send to Segments

Segments are the most common way to target large groups of devices. You may specify which segments you want to target and, optionally, which ones you don't want to receive the notification. See Data Tags (Subscription Management) for details on using tags to create segments, both programmatically and by allowing users to self-subscribe.

Programatically send notification to segments

Send to Specific Devices

You can programmatically send a push notification to a specific device based on Push Subscription ID. See User Management for details on how to retrieve the Push Subscription ID for a specific device.

Programmatically send notification to specific devices