Sometimes it'sgetting annoying to open camera in default auto-mode and then slide menu up to switch to ProMode. Here are the steps to create a short cut to launching camera in ProMode. It was implemented and tested on OnePlus 5 (OxygenOS 5.1.4, Android 8.1.0).
How to launch OnePlus mobile phone in Pro Mode by default:
1. Find OnePlus camera app among your applications
2. long press on OP camera app icon will launch dialog menu (sub-menu) with option buttons to launch portrait mode, selfie mode, video, ProMode (see screen shot below).
4. Hold down ProMode tab and drag it to your phone screen or any folder that you already created for your photo apps (see screen shot - second icon is PRO mode)
5. You are set. Test your new icon button to see that it launches OnePlus camera in ProMode