An ox of a man means a very strong man. Lencho was a hard working farmer. He worked hard in the field. His faith in God, made him to write letter to God. This shows how strong & hard working Lencho was, as he was worried about his harvest, which was destroyed by hailstorm. He thought that, writing a letter to God may help him to buy seeds for his next crop, as he didn\’t have any more seeds for his next crop. So the author compared him to an ox.