The antonym for "most odds on" can be "least odds on" or "least likely." When something is described as "most odds on," it means it has the highest probability of happening or winning. On the other hand, when we say something is "least odds on," it means it has the lowest probability of happening or winning. For instance, in a race, the horse with the most odds on is most likely to win, while the horse with the least odds on is least likely to win. However, in some cases, surprises can happen, and the least odds on can sometimes turn out to be the winner.