Gideon saw that he was the angel of Yahweh; and Gideon said, Alas, Lord Yahweh! because I have seen the angel of Yahweh face to face.
So here was Gideon hiding from the Midianites as he was threshing the wheat because if the Midianites had caught him they'd take the wheat. And so he was out by the winepress threshing wheat and here was an angel sitting under an oak tree.