‘ And it came about the same night that Yahweh said to him, “Take your father's bullock ox, and the second bullock of seven years old, and throw down the altar of Baal which your father has, and cut down the Asherah-image which is by it.” '
Now that Gideon was committed, God tested his willingness to obey, while at the same time removing from the town one of the main causes of contention between them and Yahweh. This altar of Baal, with the Asherah-image beside it, was probably a centre of worship for the whole town, as the commotion caused by its destruction demonstrated (verse 30). It serves to demonstrate how deeply the worship of Baal and Asherah had taken over as the main type of worship in Israelite towns, while they were also still observing the Yahwism rites at the central sanctuary. Yahweh was in effect being made a member of a pantheon of gods and goddesses.
It is probable that we are to see here that Gideon is being told to make use of two bullocks. The first the strongest (to pull down the altar and images) and the second the seven year old, in the prime of life, and kept hidden all those years. Yahweh had been watching over it all those years ready for this moment. Their first task would be to assist in the breaking down of the altar and Asherah-image. Then the second of the two would be offered as a burnt offering. (The first being returned to its stall). The second one was significant because its lifespan had covered the period of the Midianite raids, seven years. It would be an appeal to God concerning those seven years.
The revelation may have been in the form of a dream, or of a strong inclination forced on him by Yahweh. (LXX designates the first a ‘young bullock').
The reference to ‘the second bullock' would be clear to Gideon. It may have been the second in the stalls, or the second when put in the yoke. Or it may be reference to the fact that they now only had two because of the activities of the Midianites, the prime bullock and the second bullock. (Or it may be that ‘the second bullock' was the only one used. The verse may be interpreted either way).