On page 171, adverbs like "extremely" and "incredibly" title the sections. I think that these words have an important meaning to Oskar.
"The string pulled incredibly gently at the back of my neck..." (162)
"Extremely Depressed" "Incredibly Alone" (171)
"After an incredibly long time the door opened. 'Can I help you!' an old man asked, but he asked it extremely loud, so it was more like a scream."(152)
"I was incredibly nervous, but i maintained my confidence, and i was extremely subtle." (143)
"...Incredibly close to everything in the universe, but also extremely alone." (145)
" . . . and turned the dials extremely slowly . . ."(165)
" Then, out of nowhere, a flock of birds flew by the window, extremely fast and incredibly close. "(165)
" We went out to the street and started walking. A car drove by that was playing music extremely loudly, and it vibrated my heart." (195)
"The string between them grew incredibly long, so long it had to be extended with many other strings tied together..." (220) This quote is interesting because the title of the book is Extremely Loud and Incredibly Close. "Incredibly long" is kind of the opposite of incredibly close.
"After an incredibly long time the door opened." (152)
"I tried to think about other things. I tried to invent optimistic inventions. But the pessimistic ones were extremely loud."(235)
"I heard an extremely tiny sound. I got down on my knees again, and this time i saw that the light was off." (236)
"We were incredibly close. I could smell her breathing." (290)
There are, I think, very few examples of the word "completely" in the book. One notable use was near the end, where Oskar "felt in the space between the bed and the wall, and found Stuff That Happened to Me. It was completely full(325)." I think that since the word is used more sparingly than "extremely" and "incredibly," it is only used to signify a culmination of Oskar's feelings; that he has run out of closeness and loudness.
Oskar says, "I'm extremely emotional right now." (201)
Upon discovering that he is now meeting the stranger he saw at Abby Black's house, Oskar says to William Black, "'It's weird,' I said. 'We were incredibly close.'"(295)
Oskar uses these words to emphasize what he is saying.