One thing that defines a Green Lantern is the ‘oath’. It is the one that a Green Lantern solemnly states as he or she recharges his or her power ring in front of the power battery. In some occassions, it serves as their battlecry.
The origin of how Hal come up with this oath can be traced back to to issue #10 of the Green Lantern comic series(January-February 1962) in the story ‘THE ORIGIN OF GREEN LANTERN’S OATH’.
In the story, Hal Jordan explained to his trusted friend Tom ‘Pieface’ Kalmaku the origin of the oath. This, according to Hal, was based on three adventures he had in his first week as the superhero known as the Green Lantern: In Brightest Day: Robbers were on an attempt to prey on a Bank in Coast City wherein Hal is also present. The robbers used a magnesium bomb that sent out a very bright light that disabled everyone to see (except for the robbers that used a special goggle) during that moment, including Hal Jordan. The robbers were successful to get money off the bank, however the Green-Clad superhero was able to caught them using his power ring to act like a radar transmitter that enabled him to see the robbers’ movement despite the effects of the bomb’s intense light.
In Blackest Night: Hal Jordan learned about train robbers who hid in Sierro Hills where nobody can follow them. Hal as the Green Lantern then went to Sierro Hills to go after the robbers. He saw the robbers and followed them inside a cave wherein a black fog makes it impossible light to penetrate. He then used his power ring to make the robbers glow thus enabled him to catch them.
No evil shall escape my sight: The office safe where the company’s payroll is kept was stolen from Ferris Aircraft. The Green-Clad superhero figured out that the robbers might try to be in a distant location and blow the safe to open it without distracting attention. With that idea in mind, our superhero used his power ring to create a belt of radiation all around Ferris Company area that would detect suspicious tremors that would probably come from the robbers. The radiation then picked up something to which Green Lantern followed. He then found the robbers with the safe. After a short struggle, the Green-Clad superhero caught them.
Hal then just added the remaining lines to rhyme, thus the oath finally came to be.