"And then I said something that I wasn't planning on saying, and I didn't even to want to say. As it came out of my mouth, I was ashamed that it was mixed with any of Dad's cells that I might have inhaled when we went to visit Ground Zero. 'If I could have chosen, I would have chosen you!'" (171)
Oskar has just expressed that he would have preferred his mom to have died in the trading centers rather than his father. He obviously didn't mean to hurt her, but this almost sinister idea, which has been tucked away, just leaked out. He thought out loud. Links to death
The quote from page 171 is a very good example of a children's mind not thinking something through before saying it. No one would ever think that statement through and decide to say it out loud. It is not appropriate under any circumstance. For him to even notice it wasn't okay is a big indicator of a child's mind wandering and speaking out loud, which obviously is not always the best thing.
I think that the fact that Oskar thinks out loud has to do with the Amorphous thought process of young children. Adults don't always have to say things aloud for it to sink in, make sense or whatever, but often kids do, so this is just another way that JSF is bringing in the children's thought process.