A lot of things for QC (Pro) devices and EU Roms (Not export) are fully locked ...
Since it's needed an development / production access credentials and dig. sign by OPPO things are complexed here.
I didn't noticed the release of the DT for stable A12 yet, since I just get recently stable update 4 it (RMX3081).
OEM is part of fastboot, but HAL is missing, so only patched vbmeta and signed will do good since the Chained Partition descriptor must stay the same.
Second thing is Rooting, you can root is but still AVB (boot) must be patched and signed with oppo signed credentials.
What can help us is to DISM the app and check the comm to get possible req/send from RCSM servers.