The ox is listed first among the animals fit to serve as offerings. The reason is that the ox is associated with the sin of the Golden Calf, and God wanted to indicate that the Israelite people as a whole were innocent of that sin.

What did God see in the ox that led Him to make it primary among the offerings? Rabbi Levi said: This can be explained by means of a parable of a matron, about whom rumors circulated that she had an affair with one of the prominent men of the kingdom. The king investigated the matter and discovered that it was without basis. What did the king do? He made a feast and seated that man at the head of the table, to inform everyone that the king investigated and discovered that the rumors were without basis. Similarly, the nations of the word were saying to the Israelites: You made and worshipped the Golden Calf. The Holy One, blessed be He, investigated and discovered that their allegations were without basis [as the primary actors in that sin were outsiders who appended themselves to the Israelite people]; therefore, the ox was made primary among the offerings. (Tanĥuma, Emor)