And possibly each DSource would be represent a source database with different OS platform and Oralce version?
Also whether source DB and VDB has to be on same version and platform?
Regarding the source DB and VDB, the target and source environments do need to be on the same version, but OS version MAY vary depending on the combination. From the current (4.2) documentation:
Source and Target OS and DBMS Compatibility
The source and target must be running the same DBMS/Operating System combination (for example, Oracle 10.2.0.4 on RHEL 5.2) in order to successfully provision a VDB to the target. If the DBMS versions are compatible, the OS version on a target host can be different from the OS version on the source host.You can find more details on the supported versions, etc, in the documentation at support.delphix.com.
I hope this answers your questions?