A few ways that I have approached this common situation:
1. Add a thin strip of wood around the opening as a jamb extension.
Install casing as usual. Caulk between wall and casing if you have to shim
casing out to make miters work.
2. Put a pencil mark on the wall around the outside of where the casing will install.
Smash down the offending areas with a hammer, staying inside the pencil marks.
then attach casing with proper reveal around jamb.
3. Install the casing laying flat on the wall with proper reveals around the opening.
Caulk the areas where the casing is held away from the jamb. This is probaly
the easiest approach.
Finish carpentry is about getting the best looking results you can without having
to rebuild the house.
Best of luck with the project!
Richard The Door Guy