Complex sentence: In the novels it is always represented by Old English because Tolkien saw the relationship between Rohirric and the Common Speech to be the same as that of Old English and Modern English, which was used to represent Westron.
Split into simpler sentences: In the novels it is always represented by Old English. This is because Tolkien saw the relationship between Rohirric and the Common Speech to be the same as that of Old English and Modern English, which was used to represent Westron.