When OYPIDs are generated, there are a few things that Core+ checks for (in sequence) to generate this, which is as follows;
Firstly, it will check for an existing OYPID within the case details.
If one does not exist, it will secondly check for the OYPID field on the most recent DT_YOT_CASE_ID - note that this relates to local case transfers, not asset plus transfers.
Finally, if no value is found, it will be generated as the YP_Ref + OuCode, where the OuCode is the 4th and 5th characters of the config SYSTEMYOT.