OneSignal is a customer engagement platform: a hosted service that sends messages to the users of an app or website through several channels. The channels are mobile push notifications (short alerts shown by the phone's operating system), web push notifications in the browser, email, SMS, RCS, in-app messages and Live Activities.
A developer adds a OneSignal SDK (a code library) to an Android, iOS, Flutter, React Native, Expo or Unity app. The team then sends messages from the dashboard or through the API, sorts users into segments, and builds Journeys: automated message sequences for onboarding, engagement and retention. OneSignal AI can draft, send, test and optimize campaigns within brand rules that the team sets, and analytics report the results.
OneSignal is a subscription service with a free plan. The self-service Growth plan charges per monthly active user for mobile push, per web push subscriber and per 1,000 emails above the free amount; larger plans have custom prices.