For a super easy way of sending out a quick push, you can use Hoofpick!
From your Hoofpick dashboard, navigate to Settings - Mobile App - OneSignal Notifications.
Simply complete the fields with your message Title and Content. Click on the emoji icon to add emojis to any text box.
You may choose to include a Target URL if you want the notification to open a web browser to the URL when the user taps on it (i.e. a YouTube video). Copy the URL and paste it in the Target URL field. If you don't include a URL, the notification will just open on your app homescreen.
When you're finished, click Send and the push notification will immediately be delivered to all your subscribed app users.
This still communicates with OneSignal to deliver your message to all subscribed app users, but you don't need to login to OneSignal. Make sure your Hoofpick account is linked to your OneSignal account before you get started.