How to set ORG_ID in R12?

By using mo_global.set_policy_context
mo_global.set_policy_context has two parameters p_access_mode & p_org_id

p_access_mode:
S In case you want your current session to work against Single ORG_ID
M In case you want your current session to work against multiple ORG_IDs

p_org_id: Only applicable if p_access_mode is passed value of "S"

Syntax:
Begin
mo_global.set_policy_context(‘S’, &org_id);
End;

e.g.

Begin
mo_global.set_policy_context(‘S’, 103);

End;