Outer joins are used mostly when you want to view the records of a dimension table / fact table even if there is no join for them.
I dont think you can define outer joins in physical layer, OBIEE does not allow that way. You have to do it either in Business Layer joins or in the logical table source if you have more than one source.
Just to add to my post above, you see greyed out option for outer joins in physical layer, because probably Oracle reuesed the join definition form for physical and business layer. But that option always remained greyed out in physical layer.