First Look: Grant Gustin as Barry Allen (FLASH) on ARROW

He appears on Arrow episode 8

Here's the premise:

A seemingly impossible robbery at Queen Consolidated’s Applied Sciences Division brings Central City police scientist Barry Allen (guest star Grant Gustin) to town. Citing a similar case back home, Barry offers to help Oliver (Stephen Amell) and team with the investigation. Oliver senses there is more to Barry than meets the eye, but he’s distracted by the similarities between this current case and something that happened on the island.

source