Using the Centennial Edition, an exmaple of onomatopoeia is found on pg. 228.
There was a long, rolling clang, as though the washtub had been flung across the yard, and a confusion of angry shouts which ended in a yell of pain.
As for oxymoron, the word blackwhite is perhaps the clearest example. Since an oxymoron is a two-word paradox, it is a word which seems to be impossible, but is actually true. Here is the explanation from the novel (pg. 218).
Like so many Newspeak words, this word has two mutually contradictory meanings. Applied to an opponent, it means the habit of impudently claiming that black is white, in contradiction of the plain facts. Applied to a Party member, it means a loyal willingness to say that black is white when Party discipline demands this. But it also means to believe that black is white, and more, to know that black is white and to forget that one has ever believed the contrary.
Thus this one term represents the entire thought process evident in the novel: in order for this world order to succeed, one must believe 2 opposing ideas at once, and know that they are both true.