Both Ty Olsson and Katie Cassidy have appeared in multiple episodes of Supernatural (2005), another show filmed in British Columbia and produced by the CW.
At 12:49, in the aerial shot of "Starling City" there is a building in the exact shape of Superman's shield.
In the latter part of this episode, when Arrow is first interrogating Martin Sommers and then fights China White, you'll notice a large, tall cylindrical column in the middle of the room. This is the same set used in the season 3 episode of Battlestar Galactica, "The Eye of Jupiter" where they find the Temple of the Five. It has been redressed for this particular episode of Arrow.
Ty Olsson and Kelly Hu were both in X2: X-Men United, however, they don't share any screen time.
In the TV show 'The Vampire Diaries', Kelly Hu's (China White) on-screen daughter is played by Malese Jow, who has appeared in 'The Flash' as the character Linda Park.