What OneSignal collects
OneSignal stores a per-device push token (APNs on iOS, FCM on Android), the user's subscription status, the segments they belong to, and any tags you push (setExternalUserId, sendTag). For Journeys, it also stores event timestamps that drive the workflow.
GDPR consent
OneSignal's SDK supports an explicit consent flag: setRequiresUserPrivacyConsent(true). Until you call provideUserConsent(true), no data is sent. Your privacy policy should describe the consent gate when you target EU users.