The Old Code was a set of rules followed by knights, which were meant to maintain order in the world. It was followed strictly by Bowen. Here is the full version:
"A knight is sworn to valour,
His heart knows only virtue,
His blade defends the helpless,
His might upholds the weak,
His word speaks only truth,
His wrath undoes the wicked."
Apparently there is also a version that takes a form as an oath made when squires become knights or when someone is being sworn into office, such as when Lord Osric is sworn in as the King's chief advisor. Here is the oath:
"I, (name), choose the Old Code as my own. I am hereby sworn to valour, to speak only the truth, to use arms to defend the helpless, to use might to uphold the weak. My heart knows only virtue. My wrath will undo the wicked."