Synonyms of 'Explosion' which begin with the letter 'B' include; blast, burst, blow up, blow out, bang and bark.
A bombing?
The word 'boom' can be used as a word for explosion that starts with a b.
Another word for addition which starts with the letter N is New.
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Another word for water that starts with the letter 'O' is oxygen dihydrogen.
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Astonishing.
Atop.
advertise
Amazing
astamp
Orator is another word for speaker.