Unsurprising, ‘odds-on’ is the opposite of ‘odds-against’. In either case, odds represent the implied probability, or percentage chance, of a particular outcome occurring. For example, if you place a bet at 2/1, or ‘two to one against’, you have a 33.33% chance of winning and a 66.67% chance of losing. In other words, that particular outcome is twice as likely not to happen as happen, which is reflected by the fact that you win £2 for each £1 staked, if your bet is successful. However, if you place a bet at 1/2, or ‘two to one on’, the reverse is true, so you win just £1 for each £2 staked, if your bet is successful. Put simply, any selection that has an implied winning chance of better than 50%, which is reflected by odds shorter than 1/1, or even money, is odds-on. An odds-on selection is deemed more likely to win than lose, sometimes significantly so, such that the profit on a winning bet is always less than the stake.
By contrast, an ‘outsider’ is about as far removed as possible from odds-on betting but, beyond that, a clear, unambiguous and objective definition of exactly what constitutes an outsider is difficult. Also known as a ‘longshot’, an outsider has, at least on paper, little chance of winning a race and is, consequently offered at relatively long odds when compared with some or all of its rivals. However, ‘relatively’ is the operative word here, because there is no threshold beyond which a horse becomes an outsider. In a three-runner race, for example, the outsider of the trio might be offered at odds of, say, 9/4, while in a forty-runner race, such as the Grand National, outsiders at 66/1, 100/1 or even longer odds are commonplace.