Think of odds as a set of scales weighing one outcome against the other: this happens on one side, this does not happen on the other. Usually the two sides are close. Odds-on is the moment the scales have clearly tipped — more weight on 'it happens' than on 'it doesn't'. Point that tipped scale at a runner and you get the odds-on favourite, the one everybody expects to come first. Point it at an ordinary event — rain, a late friend, a shut shop — and you get the everyday use: it's odds on that this is how it will go.
可以想成：odds 是一副天平，一邊放「會發生」，另一邊放「不會發生」。多數時候兩邊差不多重。odds-on 就是天平已經明顯倒向一邊的那一刻——「會發生」那端重得多。把這副傾斜的天平對準一個選手，就得到 the odds-on favourite：大家一致看好會贏的那一個。把它對準日常小事——會下雨、朋友會遲到、店會關門——就得到口語的用法：這件事八成就是會這樣。