Why does Oliver have a Domino Mask painted on? So it doesn't fall off or block his vision in the heat of combat.
Oliver would later state this as the reason upon being questioned on the matter by Barry Allen in season 2's "Three Ghosts", who proceeds to design & fabricate a domino mask with those reasons as goals in mind.
According to this, in the original comics his very first makeshift "mask" was painted on with black grease.
Why doesn't Oliver have his iconic goatee from the comics? It'd be a big giveaway that he's the Arrow if they had the same distinctive facial hair.
To follow up on this, Oliver even shaves off his Perma Stubble after rescuing Walter, because Walter saw that much of his face, and might have ended up making a connection.
Ollie's Inner Monologue stops after the first few episodes. In-Universe, this is because Ollie is adjusting to no longer being alone on the island and having only himself for company. Also, he was nearly caught by Diggle speaking his thoughts aloud. Narratively, it's because Ollie now has Diggle and others to confide in and the plot is developed, so the monologue is no longer required to convey his inner thoughts.
Malcolm Merlyn's plan was to wipe out the Glades and start over. When did he cut off Tommy? Right after Tommy ran a fundraiser for the CNRI, a free legal aid clinic based out of the Glades. It would be much easier to convince the people of Starling City that they could rebuild and improve the Glades after its destruction if he were able to point towards organisations like CNRI struggling to stay open prior to the Glades being wiped off the map.
It also adds another reason as to why he closed down the free clinic his wife started - whilst Malcolm states that it was because he couldn't bear to see his wife's work in ruins following the earthquake, it would also contribute to the idea that the Glades was a black hole destroying everything good about Starling City.
Malcolm's archery skills and fighting ability originally seem to be a bit of an ass pull, but then you remember, he's a very rich man who is shown earlier in the series to be into fencing, a sport often practiced by rich socialites, as is archery.
There's also the fact that he trained for two years in Nanda Parbat, but we don't find that out until after he kicks Ollie's butt about.
Dark Archer and Hood aren't just archetype counterparts of each other. Their lives and histories are that as well. Malcolm lost his wife, an unequivocally good person, and became a dark vigilante. Ollie lost his father, an unequivocally evil person, and become a good one. Their family members who aren't in on the secret are, in many ways, angry and upset about all the secrets. And so on and so forth.
All of the stories relating to Oliver's time on the island in season one could be pretty much interpreted as Green Arrow: Year One. Though Year One stories from DC tend to be about the first year of a hero's career (and therefore imply that season one is essentially Year One), in the context of the show the island stories end up as Oliver's Year One tales. Year One's often juxtapose our hero from what we know them as to what they were before they began, and the island is considered as Oliver's true beginning.
Oliver's not a great liar but the stories he tells Felicity before recruiting her are particularly bad. Why? He's testing her to make sure she is not going to go to the police and to see if she'd be willing to help out.
Confirmed-ish in "Three Ghosts".
"State v. Queen" reveals Moira's case is unwinnable when Laurel discovers Adam's trump card against the defence. After revealing it to Moira, Moira comes clean to her children about her one-time affair with Malcolm Merlyn years prior, after which she admits it in the trial and it becomes the main focus of the prosecution's cross-examination. However, the information about the affair doesn't appear as earth-shattering a reveal as we're led to believe, maybe because it was Moira who chose to reveal it prior to the testimony... That is, until the end, when Malcolm reveals that Moira had been hiding the fact that Thea was his daughter, and therefore Oliver's half-sister. Whilst "Tremors" would later confirm that only Moira, Walter & her OB/GYN knew the truth, it stands to reason that the damaging information Adam had uncovered on Moira and would have used had he remained on the case was that the timetable for the affair made it a very real possibility that Thea was Malcolm's daughter. Out of the family's best interest Laurel focused her cross-examination on the affair alone, knowing that this fact would hurt Oliver and Thea.
This also provides a new depth to her actions after the cross-examination, particularly her reaction when Oliver tries to check if she's okay.
Why was Laurel made assistant and then lead counsel for the prosecution in Moira's case despite being a clear violation of legal ethics and protocol? Malcolm Merlyn arranged it as a backup plan to ensure Moira's acquittal. It would have created reversible legal error on appeal if he hadn't been able to sway the jury.
Oliver's insistence on a bow makes a lot more sense when you learn Shado was killed by a gun.
Barry Allen's Freak Lab Accident is turned Up to Eleven in this. First, the power in his lab shorts out, then lightning takes out a particle accelerator, and as he goes to shut the skylight all the nearby chemicals start rattling and the liquid in them starts floating up. Then a bolt of lightning basically defies natural laws when it ignores any nearby metal, water, or tall buildings and aims right for Barry, knocking him back into the shelf of chemicals. If you're going for the "Speed Force" explanation on how Barry got his powers, the whole build-up is there to make it clear that this is not a normal lightning bolt.
The comments about Yao Fei being the first Green Arrow make more sense when you take into account Ollie shooting Roy in the leg to protect him and stop him from investigating Mirakuru, and later removing the arrow himself. It has come full circle from Yao Fei shooting Oliver and then nursing him back to health.
Isabel is working for Slade. Summer Glau is working with the original Terminator.
In "City of Blood" Felicity reveals that she worked her ass off to avoid becoming rich men's eye candy, which explains why she got so pissed when Oliver made her his EA. Everything she had ever done was undone in that one move.
The familiarity Felicity displayed with card counting in "The Undertaking" makes a lot more sense once we found out in "City of Blood" that she grew up in Las Vegas.
Thea's main character flaw throughout the series is her vindictiveness and holding long, sometimes unjustified, grudges over perceived slights. Those are perfect traits you'd find in a villain.
Mirakuru has, so far, been demonstrated to make you near unstoppable, and so far Oliver has only just managed to kill Cyrus Gold and by his claim, killed a Mirikuru-powered Slade on the island, only for it to be revealed that no, he didn't kill them. Cyrus Gold, in the comics, is better known as Solomon Grundy, and his primary power is that he comes back from the dead every time he's killed. In other words, he could still be out there. Then, lets not ignore the fact that Mirakuru apparently does leave those who take it completely unkillable, and suddenly, the already scary people who take it become even more frightening.
Lightening the mood here, Grundy's often very different in power levels, intelligence, and personality when he comes back. This includes becoming considerably nicer. We already know the drug affects the mind.
Slade isn't an archer. So when he puts arrows through peoples' eyes, does that mean he manually shoves it in?
Slade mentioned in season 1 that he has a son, and from the looks of things he's been too busy with his revenge on Oliver to even consider seeing him again. Imagine having your father go missing for six years, and then turn up in a different country as a wanted fugitive who kidnapped one woman and murdered another. Now, remember how Slade used to be, and how his son likely remembers him, to how he is now; if he ever sees his father again, lil'Joe Wilson is NOT going to like what he sees.
Made worse given the Mirakuru thing: Slade believes he's on a mission to avenge Shado, and her memory haunts him in a manner that makes it apparent he sees her egging on his more violent actions. Given the way he reacted to having to kill Moira, and his own word about not being able to let it go as much as he wants to, it's very likely that he's tried to go home and reunite with his son, but his hallucinations of Shado have basically forced him to withhold doing so.
Sebastian Blood killed his father because he was abused by him. Think back to when we met his mother, and how she called him "the devil" for doing so. Meaning she herself was abusively neglectful to him, possibly because of earlier abuse from Blood's father.