When Oliver is chasing the drug dealer in the flashback, a billboard can be seen behind him that reads "In brightest day, in darkest night, come to Coast City when money's tight". This is an obvious play on the famous Green Lantern Corps oath; "In brightest day, in blackest night, No evil shall escape my sight. Let those who worship evil's might, Beware my power, Green Lantern's light!"). Coast City is the home of Hal Jordan, the most popular Green Lantern in the comics.
In the flashback scene in the bar with Amanda Waller and Oliver, Waller says, "The world is too small for someone like Oliver Queen to disappear." In Christopher Nolan's Batman Begins (2005) Liam Neeson's character Ra's al Ghul says to Bruce Wayne, "The world is too small for someone like Bruce Wayne to disappear." Both of these characters are part of DC Comics.
At the beginning of the scene where Amanda Waller and Oliver Queen are at a bar, you can briefly see a man wearing a green pilot's jacket with the name "Jordan". This is a clear reference to arguably the most popular Green Lantern, Hal Jordan, who was a pilot in the Air Force.
Quentin Lance mentions that Star City should have a 'Flash Day', just like Central City, in an effort to draw more tourism, a direct reference to The Man Who Saved Central City (2015), and once more aligning the two show's timelines.
Kord industries appears in this episode, which is the company of another DC hero, Ted Kord aka The Blue Beetle