I had to perform the following Steps to get OpenCamera on lockscreen:
Enable "Settings -> Security -> Allow camera access when device is locked".
Enable "Settings -> System -> Gestures -> Quickly open camera"
To select another camera app:
"Settings -> Apps -> Camera -> Open by default -> Clear default preferences"
"Settings -> Apps -> OpenCamera -> Open by default -> Clear default preferences"
Lock device, try camera shortcut, unlock and select your camera app, select "always".
In OpenCamera I had to enable "Settings -> More camera controls -> Show camera when locked".
Go back from settings to OpenCamera main screen.
Try it, works. Maybe this is helpful.