In episode 1x5, "Damaged," Oliver wears a prison shirt with the number 399471 on the pocket. This is the hex code for the shade of green of his hood.
Oliver Queen has an occasional "tic" described by Stephen Amell as a sort of anxiousness when Oliver wishes he was holding his bow.
John Diggle was created specifically for this series, and was added to the mainstream DC universe starting with the New 52 titles, introduced in "Green Arrow" Issue 24, October 2013. He joins Chloe Sullivan (Allison Mack) from Smallville (2001).
The Queen Mansion exterior shots are filmed at the Hatley Castle in the Hatley Park National Historic Site located in Colwood, British Columbia. This is the same Castle used for the Luthor Mansion exterior shots in Smallville (2001), and for Professor Xavier's castle in the X-Men (2000) film series. The location is also used in Deadpool (2016) and Deadpool 2 (2018).
Midway into Season 2, the character of scientist Barry Allen is introduced. Soon after his debut he suffers a classic comic book "freak accident" and becomes 'The Flash' in his own TV series The Flash (2014), which would go on to crossover with Arrow a number of times.