Hi Hasan:
If I tie HOE Pri Ledger with just one OP Unit thru MOE%OPER%; then how I am going to enter journal entries for HM SW OU ? I try using MO%OPER% at resp level blank and instead use MO%SECUR% at resp level entering MOAC security profile; in which case entering journal entries with tax required option giving error.
But except for journal entry, other things work fine with MO%SECUR% profile entry like I could see all my operating units I gave access to the responsibility. But how to tackle the above issue - if you could please tell ? Thanks.

You enter journal entries in a ledger, not in an operating unit.
In reality you do not even have to tie a ledger to any operating unit, to be able to enter journals.
One ledger could be tied to one or more operating units. That would mean, all those operating units would send data to the same ledger.
Transactions in sublegers like AP, AR, etc are entered in operating units. Transactions in GL is entered in ledgers.

If you are talking from taxation perspective (as you have mentioned in your subject), then yes MO Operating Unit profile option is required for GL responsibility if you wish to calculate taxes at GL level. The tax codes are derived based on this profile.
MO Security Profile is defined for the GL responsibility if you wish to control the security on drill down. But then you also have to enable the profile option "SLA: Enable Subledger Transaction Security". If the security is not enabled, then you can drill down to any subledger transaction. If the security is enabled then you can only drill down to those operating units that are part of the security profile attached.