For recommended OD values, see: Ranking criteria
Overall difficulty (OD) defines how difficult it is to achieve high accuracy on a beatmap. The value ranges from 0 to 10, where higher OD will require more accuracy and precision. Since accuracy is important for gaining health, overall difficulty also influences how hard it is to pass a beatmap.
Higher OD values mean shorter time windows to clear hit objects, both in general and in terms of getting high score values. The width of the hit window, centered on the hit object's correct time, is defined by the following table:
If the player hits outside of the 50's hit window, it will count as a miss.
In osu!standard, sliders will reward a 300 as long as they are hit within the 50's hit window. This is sometimes referred to as slideracc and is removed in ScoreV2.
Overall difficulty also affects spinners, in that they must be spun more to fill up the gauge in time. In osu!taiko, the denden will also need more hits to be cleared. The spins per second required to clear a spinner is defined by the following formula:
5 - 2 * (5 - OD) / 5
5 + 2.5 * (OD - 5) / 5
In osu!standard, the timing system will not allow a hit object to be cleared until the previous object is cleared (or missed after the hit window passes). This can lead to instances of notelock where players will miss a note even though they hit it perfectly in sync with the beat. This occurs more often on beatmaps with low OD values, because the hit windows of hit objects overlap more frequently.
There are four mods that alter the overall difficulty when activated:
While Half Time and Double Time do not change the OD value, the speed difference leads to a change in the hit windows. Because the scaling is different for each score value, DT causes the windows for 100 and 50 to become tighter than usual compared to 300, and HT causes them to become more lenient.
Overall difficulty is visible when viewing osu!catch beatmap information, but it does not affect gameplay.