Image calibration is largely the same whether you're working with OSC or mono. If you think of a OSC as just a mono camera then it makes it a lot easier
It is best if all calibration is done without debayering. A Master Dark is just stacked raw subs like you would with a mono.
Making a Master Flat is the same, calibrate your flats and then stack without any debayering.
So, debayering is the step between calibrating your lights and registering.
- Calibrate your Lights
- Debayer
- Register
- Stack
As for your second question, calibration frames should always be as close to the lights as possible. Having the same gain, offset and temp is the same regardless of whether you have a mono or OSC, CCD or CMOS.
The Offset is basically your bias level so as long as you're getting bias frames somewhere between 200-1000 ADU you're doing good.
What camera are you using?