Could you check if the "Push Notifications" capability is turned on? If it is not, turn it on and see if it works, otherwise keep reading.
Make sure you're running the latest version of the OneSignal SDK (2.1.14)
as this version should have resolved the issue. Next, try the following fixes:
Fix 1
From the navigation panel, select your top blue project. Then select your target and navigate to the Build Settings. Search for other linker flags and add the following entry: -ObjC
Do a project clean and run again, see if it works.
Fix 2
In your bridging header (if you have one), or on any header file that is visible to your App Delegate, create an extension for OneSignal class in which you define the following interface method:
+ (void)didRegisterForRemoteNotifications:(UIApplication*)app deviceToken:(NSData*)inDeviceToken;
Next, override the didRegisterForRemoteNotifications
delegate method in your app delegate and call the above method on OneSignal and pass both parameters to the SDK.