When you are at that client site that insists that you must use your own account to interact with PeopleSoft but they don’t set your schema, just change your default schema using the command:
alter session set current_schema=sysadm;
Where sysadm is the default PeopleSoft schema in Oracle, however, PeopleSoft does not have any issues with the schema owner being different then sysadm. A couple of ways to find the schema, is to look at the table: psdbowner (ownerid) or psstatus (ownerid). By default the psdbowner table is stored in the PS schema.