In addition to FinancialWar's correct answer, I'd like to mention that words that are all ON readings largely fall into two groups, words derived directly from the Chinese, and words where the kanji were put together in Japan using ON readings to create a new word (in which case the Kanji determine the pronunciation). That's not quite the whole of ON reading words, there are other cases where the ON readings are fit to an originally Japanese word (called 'ateji'), but they are relatively rare in modern Japanese.