Well, with only a little Googling I found that Fernando was a Portugese prince, who later became a saint. Given that there is a "San Fernando Valley", "San Fernando Mission" etc. in the Americas, and that the Portugese explored much of them, it isn't a leap of imagination to see San Fernando as a patron saint.
Oak leaves are a symbol of faith, strength and virtue (usually military) in classical art and history, just as the laurel wreath was given to victors in battle and the Olympic games. Adding oak leaves to a decoration is to recognise a degree of achievement over the normal - the American Purple Heart honour can likewise be upgraded with a palm leaf.
I don't think Hergé is parodying
per se, just using the language of military honours...
Update: And I prove myself (possibly) wrong! Spain has as its highest military honour the
Laureate Cross of Saint Ferdinand -
that could be Hergé's inspiration.
