“Ollie ollie oxen free” was a catchphrase used in children’s games such as hide and seek, capture the flag, and kick the can to indicate that players who are hiding are now “safe” come out into the open without losing the game.  A probable origin of this call to safety is a German phrase, “Alle Alle Ochsen Sind Frei”, meaning “all the oxen are free”, signifying the Oxen could run from their captors.