I've taken both exams. When I took the CDEI, it was part of the beta testing process and we were not allowed an additional reference.
When I took the CDEO, it was in person & paper/Scantron. We were allowed to use the Clinical Documentation Reference Guide.
It's possible that the reference guide is now built into the exam tools for the CDEO. My understanding is that the electronic exams have many of the tools built into the exam. If you're looking to take the CDE electronically (whether in person or live remote proctor), I would ask whether there are any reference materials available within the exam.
I have a feeling that may be the case, but I do not know for sure.