In this blog post [ Oracle12c Upgrade ] there is a statement concerning the Oracle 12c SAP user:
"This step is optional but make sure that Oracle12c installation is done with user oracle"
As it was a bit confusing, I tried to comment, but as this is not allowed I use this thread. Another blog mentions the required user ID:
<oracle> user is not must for 12.1c single instance
Because of "buried" shell scripts and other required rework/risk, the original restriction was relaxed.