Hi Satish
Organization Reassignment Action is generally used when the employee gets transferred, gets promoted etc. This action allows changing IT0001 for the employee where you would be able to change his position, job, personnel subarea, payroll area etc. Along with IT0001 you are also allowed to change IT0007, IT0008 etc.
When you execute this action in PA, the changes get reflected in OM as well provided your PA+OM integration switch (PLOGI + ORGA) is ON in table T77S0.You will not need to make any change in OM separately after executing this action if this intergration switch is on.