The angel of Yahweh came, and sat under the oak which was in Ophrah, that pertained to Joash the Abiezrite: and his son Gideon was beating out wheat in the winepress, to hide it from the Midianites.
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.