Oly Oly Oxen Free
Oly Oly Oxen Free is a six-note, six-beat tone used as an all clear code by the SPARTAN-IIs. They learned it from Deja on Reach when they were younger and have been using it ever since. Spartans would either just whistle the tone or would whistle it and shout "Oly Oly Oxen Free" after.  The proper counter-response was "Oly Oly Oxen Free, All out in the free, We're all free."
Oly Oly Oxen Free was a phrase once used by children playing hide-and-seek, which meant that the game was over and they could safely come out of hiding. Centuries later the phrase used by the SPARTAN-IIs as a secret codeword to signal their presence. Only a select few outsiders knew it, who were close to the Spartans' "family", knew of its use, such as Dr. Catherine Halsey, Cortana, and Sergeant Avery Johnson. Thus, the signal was extremely important to each Spartan; they would rather die than give up the significance of the tone, which could potentially be used to betray or endanger their teammates. They learned this tune/code from their AI instructor, Deja, while they were training on Reach.
The term Oly Oly Oxen Free was used on numerous occasions throughout the Human-Covenant War. Some of the times it was used are as follows:
- After the Spartan Team had beaten Tango Company and John-117 had checked to make sure their evacuation point was all clear.
- After the Fall of Reach, Anton-044 used it as an S.O.S. signal over the E-Band. 
- In order to allow the Spartans to enter CASTLE Base, Kelly-087 used it to let Dr. Halsey know it was them. 
- During Operation: FIRST STRIKE, Linda-058 used it to let John know where she was so he could find her and pick her up. 
- At the end of Operation: FIRST STRIKE, Sergeant Johnson used it to let Blue team know that the Gettysburg was holding station behind a nearby moon. 
- When the rest of Blue Team, minus John-117 arrived on Onyx, Kelly whispered "Olly Olly Oxen Free[sic]", over the COM. 
- When Dr. Halsey took Kelly-087 on the Beatrice with her.
- Oly Oly Oxen Free was originally used to tell players in a game of hide and seek that the game was over and they could come out of hiding.
- The phrase is spelled "Oly Oly" in Halo: First Strike but "Olly Olly" in Halo: Ghosts of Onyx. The latter spelling is the grammatically correct, but the former has been used more.
- The Marines use it on the Halo 3 level "The Ark" when firing on the first Covenant encampment, as well as in Crow's Nest, but only if the IWHBYD Skull is activated.
- Dana Awbrey alluded to this phrase by titling a blog post "Oly Oly Axon Free".
- Oly Oly Oxen Free has 7 syllables.
- ^ a b c d Halo: First Strike, page 166
- ^ Halo: First Strike, page 56
- ^ Halo: First Strike, page 118 ("Or maybe you're just not saying anything they want to hear," Kelly said. She whistled a six-note singsong tune.)
- ^ Halo: First Strike, page 322
- ^ Halo: First Strike, page 326
- ^ a b Halo: Ghosts of Onyx, page 236
- ^ Halo 3, campaign level, The Ark
 See also