With this tutorial, you can easily change your default launcher with the stock OnePlus launcher. You just have to be rooted and should have Android 9 on your device for this to work. Also, you would need Magisk installed along with the Magisk Manager installed.
the team behind the Lawnchair launcher app released QuickSwitch, a Magisk Module that allows for rooted Android Pie users to change the recent app overview provider from the default implementation to a supported third-party launcher. Of course, Lawnchair itself is compatible with this module, but a developer has modified the stock OnePlus Launcher as found on the OxygenOS 9 builds for the OnePlus 5/5T/6/6T to be compatible with QuickSwitch.
One this particular launcher setting wallpaper through the launcher is buggy but you can set a wallpaper through an app of your choice or through the gallery.
If you like to change the launcher or delete OnePlus launcher please do select your launcher through Quick Switch first and after a reboot, change the default launcher.
Step 1. Download and flash Quickswitch Magisk module either from the TWRP recovery or through Magisk manager (Recommended).
Step 2. Now download and install the modded APK of OnePlus Plus Launcher. At this stage please do not choose it to be your default launcher yet.
Step 3. With your default launcher open Quick Switch.
Step 4. Now select OnePlus launcher as your recent apps provider.
Step 5. Reboot the device.
Step 6. Now select “OnePlus Launcher” as your default launcher.
Source: XDA DEVELOPERS