Oliver guesses that Barry has saved Central City 170 times, and is corrected that it's actually 180. The two figures are in-joke allusions to Arrow (2012)'s total episode count and this episode being The Flash (2014)'s 180th.
Oliver says his iconic line "You have failed this city!" for the very last time in the Arrowverse.
When he greets Barry in limbo, Oliver admonishes him by saying, "What have you done now?", a callback to his line "What have you done this time" in "Elseworlds, Part 1 (2018)" when he realizes that he's living Barry's life.
When Oliver talks to Diggle about his rejection of the box that came to him from Space (strongly implied to be a Green Lantern ring), Oliver tells Dig that his Brightest Days are still ahead of him. This is likely a tongue-in-cheek reference to the Green Lantern oath, which opens with the line: "In brightest day, in blackest night, no evil shall escape my sight."
This marks the long awaited return of Oliver Queen/Green Arrow after his death in Crisis on Infinite Earths.