How would you classify the reaction #2PbO + O_2 -> 2PbO_2#?
You could call it either oxidation or a synthesis (aka "combination") reaction, as it fits both categories.
Any time a chemical reacts with oxygen, the process is correctly called oxidation.
However, I suspect you are trying to fit this into one of the categories - synthesis, decomposition, single replacement or double replacement.
This one is synthesis, as it follows the general form of
In other words, two reactants form one product. This is synthesis.