Record of schema related changes - Oracle
That's exactly the kind of thing that oracle AUDIT is created for. The basic view is DBA_AUDIT_TRAIL, though there are several others.  By defalt, Oracle has some auditing already enabled, but you may want to review and modify. Also you want to use the dbms_audit_mgmt package to move the audit tables out of the SYSTEM tablespace into their own dedicated tablespaces, as well as to do normal housekeeping.  Details are in Part VI of the Security Guide.