The ironborn are living constructs and are immune to mind-affecting spells. The oaths don't fit them very well as two provide resistance to enchantment and one to curses, diseases, and poisons. I also rejected Oath of Protection as nearly impossible to ever use.
The current oaths don't support an NPC who will in combat against damage-dealing PCs. I'm not sure if this design choice is deliberate. A bonus +1 to Reflex checks against spells would be useful (Oath of Hope--avoid some type of failure like a failed save) or initiative (Oath of Honor--draw first blood to activate and show honor). Just throwing some more useful options out for oaths.
Also, the mount is a great idea but any cavalier inside a structure at 3rd-level loses a huge class benefit. While the text says the cavalier is powerful without a mount, a 3rd-level cavalier loses two class abilities while inside a small structure that doesn't allow mounts.
If the charge ability worked for the cavalier (while mounted or not) it would be useful but perhaps overpowered. Perhaps a reduced charge bonus while on foot for all the charge class abilities?