Mood Whiplash - The third book, in contrast to the previous two, is almost entirely dramatic and much less comedic. Of course, if we work in reverse, the first book whiplashes into the second - as the first book was almost entirely humor in comparison.
If you never thought cheesemaking could be epic, this'll change your mind.
As well as a perfectly ordinary game of one-on-one basketball beetween two children...one of which is a superpowered cyborg, the other given similar powers by his own Powered Armour. Cha.
Not allowing the heroes to top them, the ODA also manage to break out this trope - Humongous Mecha teddy bears of death? Exploding teapots that actually cause a main character to be Killed Off for Real? Check and check.
Running Gag - Arctic foxes are a running gag in the first book. Similarly, anyone who mentions the King of Switzerland (who is in fact the ruler of the Hollow Mountain, which is located in Switzerland - not the country itself) can expect to get the reply "Switzerland is a republic!" People in the series also have a tendency to confuse zeppelins for blimps, which is usually corrected by either Parveen or the narrator.
Hamish X seems to have a thing for toast. He requests it to be made by George in the second novel, and in the third...let's just say it's made awesome.
In addition to the above, there are several other examples: the Cheese Pirates (as well as the cheesemakers) have an obsession with cheese; the ODA Agents seem to enjoy having both honey and sugar in their tea, implying that they like things that taste sweet.
And Parveen loves porridge, which seems to fit his role as The Stoic.
We Can Rebuild Him - Hamish X, who was actually the victim of a staged killing by the ODA, drowned, and then brought back to life as a cyborg. Yes, these books are meant for children. Yes, they're that awesome.