Yes and no.
I'm statting up an elven cleric/ranger, with a longbow, and have been mulling over the same question. The rules are pretty clear:
(1) you can't take an OA with a ranged weapon
(2) you can take an OA with an unarmed attack
(3) unarmed attacks do not have to be made with the hands
(4) so, an archer can make an OA with a well-placed kick or headbutt.
It's not going to do much damage. A 'one-handed' unarmed attack does 1d4, which I reckon covers both kick and headbutt. But even 1d4 will do the trick against a minion.