Shouted by children in the United Kingdom and the United States to signify that a game of hide & seek or similar is over, and that all players can come out of hiding.
The original form of the phrase is hypothesized to have been 'all in free or all's out come in free', with time and repetition distorting it eventually to 'all-ee all-ee (all) in free' or 'all-ee all-ee out(s) in free'. The additional "ee" sound and the repetition of "all" contribute to audibility and rhythm.
From this root, a number of variant folk etymologies come forward, the most common of which has 'oxen' replacing 'out(s)' in, giving 'all-ee all-ee oxen free'; with the 'all-ee' reinterpreted as the diminutive nickname 'Ollie'.
Accurately dating the expression is difficult, as it wasn't collected until the 1950s and later. However, it seems that they were in common use by the 1920s, and probably earlier, as the expression "home free" is found in print in the 1890s and the game of hide-and-seek is at least 400 years old.
NB: a scar faery notes that in the Black Country, semantic drift has transformed the familiar refrain: "There I was," she writes, "casually looking over the children's games node, when it suddenly hit me that Ali ali oxenfree -- or ollie ollie oxen free -- was exactly the same as what I knew as "acky acky 1 2 3". Drift is a magical thing."