John Diggle is named after Andy Diggle, a Green Arrow writer. Later, Andy Diggle is established as the name of John's brother.
A crooked judge named Grell is a nod to Mike Grell, an acclaimed writer/artist of the Green Arrow comics.
One woman wronged by Adam Hunt, Danielle DiDio is a nod to DC Comics co-publisher, Dan DiDio, another is named for science fiction writer Nancy Kress.
Episode 2 has Victor and Emily Nocenti, who are named after comic writer Ann Nocenti.
One of the places Deadshot has done assassinations is Corto Maltese, a fictional island that plays an important role in Frank Miller's Batman: The Dark Knight Returns. Corto Maltese is also an acclaimed comic books series by Hugo Pratt, making this a shout out to a shout out.
Deadshot has his hideout at a motel called Bates Motel.
A meeting takes place at "Faquet-Lemaitre" Consulting, a reference to set designer Dominique Faquet-Lemaitre.
On the island, Oliver partners with the Australian agent Slade Wilson, resulting in:
A very fast blink-and-you'll-miss-it in episode 13 "Betrayal". When Cyrus captures The Hood (for all of 3 seconds), he quips, "Lose the bow, Merida." The humor of which is probably completely lost on Oliver since he wasn't around for the movie.
The family of bank robbers who escape their robberies via a hole in the ground, and force to dress in their outfits in order to confuse the police as to which one are the robbers is one to Inside Man, which used similar tactics, albeit the hole in the ground was for a different purpose.
In Unfinished Business, the Count, insane from an overdose of the drug Vertigo, is rambling a line from the movie Vertigo.
The Undertaking contains a shot during one of Oliver's rampages that is almost certainly an homage to the infamous hallway sequence in Oldboy.
In "Sacrifice", Malcolm Merlyn is disabled by catching an explosive arrow and having it explode in his face, like Loki in The Avengers.
William Tockman, the "Clock King", is reinvented in "Time of Death". Dispensing with the silly costumes he's worn in the past, this incarnation sports round glasses, a leather jacket, and a shirt buttoned up to his neck. This, in conjunction with his background as a terminally-ill scientist Anti-Villain, gives him more than a passing resemblance to Walter White.
The Clock King also quotes War and Peace at a mook who nearly messed up their previous heist.
When being taken out of his cell, Shrapnel quotes Chekov about corporal punishment.
"Three Ghosts", aired just before Christmas, is a Shout-Out to A Christmas Carol, as Oliver hallucinates the titular ghosts throughout the episode: Shado is "Past", and the horrors of his time at the island; Slade is Present, representing his current crisis of confidence and the Big Bad of the season, as revealed at the episode's end, while Tommy is "Future", and the hope for Oliver that he can, in fact, become a hero.
"Legacies" name-drops Keystone City and Coast City, the home towns of The Flash and Green Lantern respectively.
In the first episode of the second season, "City of Heroes", the Chinese pilot of the plane Diggle and Felicity are on is played by Stephen Chang. He is also credited with voicing a Chinese pilot in the game City of Heroes.
In Corto Maltese, it's revealed that Thea took the identity of Mia, the character she's based on.
Felicity's assistant is named Conway, after the comic writer best known for killing Gwen Stacy at Marvel.
In the episode "The Secret Origin of Felicity Smoak", Cooper's shirt features Starro in the same pose as the famous Justice League cover. Felicity's Goth Girl look in college seems to be based off Death from The Sandman, complete with ankh necklace.
Towards the end of "The Brave and the Bold," Oliver says that "Digger Harkness is now cellmates with one Slade Wilson." Making Ashur and Crixus cellmates is not a good idea.