Hello,
I need to ask that as you have said in the tutorials that setting up business group is mandatory to setup
operating unit, Business Group is the part of HR module in R12, now if some Company don't have HR module then
what will be the scenerio, will the default Business Group be there with financial modules, and if not how will
we setup the Operating Unit.

Thank You

Creation of Business Group is not mandatory as a default business group is there. System comes with one business group. You can use that. However an operating unit cannot be created without a business group existing already.
That is one thing. The other point is how to create or modify a business group if a company is not using HR.
Even if a company is not using HR, "shared HR" is always in use. Organization screen is owned by HR, but the screen is available to most of the other responsibilities as well (like Employees screen). Organization screen is visited in almost all implementations, not only create create business group but also to create other organizations like operating units and inventory orgs etc.
Also keep in mind all modules are installed automatically when you install E Business Suite. I have covered this topic several times in my videos. It is not a matter of the module is there or not, it is more of a matter of, is the module marked as "in use" via licence manager or not. Hope that helps.