In a discussion with Sam, Diggle makes a passing mention of the glowing box he came across in the Arrow (2012) series finale, the contents of which was implied to be a Green Lantern power ring.
In 2009, Tyler Hoechlin and David Ramsey guest-starred in the same season 2 episode of Castle (2009).
Although they didn't share any scenes, David Ramsey and Dylan Walsh were both on Blue Bloods (2010) and both played the Mayor of New York City.