Hello Arjun,
You did it with the OutOpportunityID. 
Insert into the EventHandler Initialize before reading the BO a DataOperation in which you assign / transfer the unbound, inport parameter /Root/Inport/OpportunityUUID to a new field whioch is bund to the ID of the Root node of the Opportunity BO.
Afterwards use this element to read the data.
HTH,
Horst