Which fuels (burning materials) can be used in a wood fired oven?
Regarding the use of pallets, the link below is to a site that gives good examples of why not to use them for any type of repurposed project due to unknown chemical treatment, e-coli contamination, mold, fungus and other nasty things that could leach from the wood.  

We use hardwood branches from known (chemical-free) sources for smoking meat.

http://www.organicauthority.com/sanctuary/repurposed-wood-pallet-toxic.html

Here is an additional link to a pizza oven manufacturer's site with their wood recommendations.  They do not recommend using any wood that has had any sort of chemical treatment.  If you do not know the source of the wood, there is no reliable way to verify that it is chemical-free.

http://www.fornobravo.com/pizza_oven_management/choosing_wood.html