BattleOdds is a ratio of the total combat strengths of the 2 armies. A ratio of 5:4 indicates that your army is 25% stronger than your opponent's. In spite of the name, it cannot be directly related to the odds of winning a battle, though the correlation should be clear. One reason why this is so is that BattleOdds does not take into account the opposing generals' battle stars. It also can't account for terrain effects. Here is a link to a meandering discussion on the subject, including a post from CA's JeromeGrasdyke.