How can we "scalar extend" both arguments?
3 4 5 (-⍤0 1) ⍳5 ⍝ Same as ∘.-
3 2 1 0 ¯1
4 3 2 1 0
5 4 3 2 1
3 4 5 (-⍤1 0) ⍳5 ⍝ The axes are reversed!
3 4 5
2 3 4
1 2 3
0 1 2
¯1 0 1
Outer Product is simply the choice to extend both arguments in turn, along with the choice to put the axes of the left argument before those of the right.