Hi,
One option you have at WBS element is choosing Project Type selection- as you wanted to have both CAPEX and OPEX in one project structure.
Ideally speaking, i will never suggest to have both types in project structure rather would like to have to two different project structures.
Responsible Cost Center field can be made as a mandatory feild to provide CTR information.
You can validate Project type for setting up validation.
Thanks,
Sudhakar