Monthly Updates (What's Next)
Before your OnePlus phone gets an update to Android 9 Pie you'll probably get a few other software updates. These are either Android 8.1 Oreo or monthly security patches.
OnePlus is great when it comes to keeping devices secure and safe. They recently delivered a big July Android Oreo update to multiple phones, then another in August. Then similar updates or betas in October, November, and December.
OnePlus is great at taking those moments to add new features, improve the camera, or do other small things the community wants.
Basically, while you're waiting for Android 9, don't get upset if you get an update (or even 2) and it's not Android Pie. The company will continue to deliver monthly security patches alongside the beta program as they work towards pushing out Android 9 Pie to every compatible device.