"Hundred to one" is a phrase that denotes a very unlikely event or outcome. The opposite of this phrase can be "even chance," which suggests that the probability of an occurrence is 50-50. Another antonym for "hundred to one" can be "certainty," which means that the outcome of an event is guaranteed. Additionally, "very likely" can be considered an antonym, implying that an event has a high probability of occurring. "Probable" and "expected" are also antonyms of "hundred to one," indicating a likelihood of an event happening. Therefore, depending on the context, different antonyms can be used for the phrase "hundred to one.