The following contains spoilers for Justice League: No Justice #4 by Scott Snyder, James Tynion IV, Josh Williamson and Francis Manapul, in stores now.
Since DC Comics rebooted its universe with the New 52, Green Arrow has been held one notch below the Justice League. More than that, he's proved to be a massive irritant to them on multiple occasions. Even when he decided to sign on as a part-time member of the League and got caught up in the madness of Dark Nights: Metal, he remained on the fringes rather than the front lines.
But Ollie's never really minded, and even has admitted that he wouldn't be able to gel with the team's heavy hitters since he's used to operating on a street level. But things changes, and since Brainiac sent the Justice League to space in No Justice, Earth has found Oliver Queen to be her biggest defender.
Thanks to the machinations of Amanda Waller, the person in charge of the Suicide Squad and other metahuman Task Forces, the same cosmic energy trees the League battled on Brainiac's home planet were allowed to fully grow on Earth. Worse, the Omega Titans have landed on Earth as the final issue of No Justice opens.
While Waller wants to use her SAT phone to nuke one of the trees from orbit, Ollie was quicker and managed to get a call to Hal Jordan and the rest of the human Green Lanterns, who had been working on repairing the destroyed Source Wall. This proved to be the right move, as the Lanterns came to rescue just as Brainiac's son Vril Dox was about to murder Waller and Ollie.
After the Justice League successfully charges the seed Brainiac planted on Earth with all four cosmic energies (Mystery, Entropy, Wonder and Wisdom), Hal constructs a giant green gun to fire the seed into the Entropy Titan so its siblings will eat each other instead of Earth.
And since the fate of the world is at stake, there's no one more fit to pull the trigger than the Emerald Archer -- after the construct is appropriately converted into a giant green crossbow, naturally. His aim true, Ollie hits the Entropy Titan and the other Titans turned on their sibling, vanishing from Earth without a trace.
This is when things get really interesting for Oliver.