Based on the Oracle Database 12c Certified Master Upgrade Exam (visited 2018-05-03 - no longer valid). I rate myself as follows (1=poor to 5=good).
The new version (2020-01-07) is here.
According to Oracle Database 12c Certified Master Upgrade Exam, the exam environment is:
Build diaries for system used exclusively for OCM 12G can be found at 12C OCM System Builds