Advertisements have a time and a place, and the notification tray should be off limits. Several companies have shamefully used advertisement notifications before, including Samsung and HTC. OnePlus has had quite a few mishaps over the past few months, but that hasn't stopped the company from pushing ads and surveys to its own devices.
The earliest instance I can find of this behavior was about a month ago, to promote the OnePlus 5 launch. I can almost let that one slide, because existing OnePlus phone owners would probably be interested in the latest model. But the latest push is not okay at all - it advertises a survey that users can take to possibly win a OnePlus 5.
The notifications are sent by the "Push" system app in OxygenOS. One Reddit user pointed out that you can block notifications from that application in the system settings, but that was from a month ago, and something may have changed since then.