You can start NOW!

LC, 140 and 485 can be filed BEFORE you start working for the employer because employment-based immigration is for future employment.

As my personal experience, my employer started LC for me before I started working for it. A few months later after LC was filed, I joined the employer.

It has nothing to do with H1B approval or OPT. Your employer can start LC when you use OPT. LC is a property of the employer and it has nothing to do with your immigration status.

So don't waste any time, just get started now.