I have installed a new camera app that I'd like to use by default. I set it as the default app in the settings, but that didn't make it the target app for the lock screen shortcut or the O gesture.
The most simple method - and I've tried this on my own OnePlus 3 running OxygenOS Open Beta 21 - is to simply disable the stock camera app - known as "OnePlus Camera" on OnePlus devices.
To disable it, go to
Settings > Apps > OnePlus Camera > Disable
That should do the trick.