Title: Flash vs. Arrow
Content:  The episodes, along with the rest of The Flashs first season and Arrows third season, were released separately on Blu-ray and DVD on September 22, 2015. Bonus features include behind-the-scenes featurettes, audio commentary, deleted scenes, and a blooper reel. The episodes became available for streaming on Netflix on October 6 and October 7, 2015, respectively.
Title: Flash vs. Arrow
Content:  The first indication of potential crossover events in the Arrowverse occurred on The CW during the 2013–14 television season, when Barry Allen was introduced in the eighth episode of Arrows second season ahead of the debut of The Flash. In July 2014, it was announced that the eighth episodes of the third season of Arrow and the first season of The Flash would be a two-hour crossover event. The crossover was originally planned as the seventh episodes of each series, but was pushed back due to the large amount of work needed to accomplish it. In particular, the schedule coordination of trying to "jam another episode into the 23-episode schedule for 
Title: Flash vs. Arrow
Content:  "Flash vs. Arrow" is the inaugural Arrowverse crossover event, broadcast on The CW, featuring episodes of the Arrowverse television series The Flash and Arrow. The event began on December 2, 2014, with The Flash episode "Flash vs. Arrow" and concluded the next day with the Arrow episode "The Brave and The Bold". The crossover sees Team Flash (Barry Allen / Flash, Caitlin Snow, and Cisco Ramon) helping Team Arrow (Oliver Queen / Arrow, Felicity Smoak, and John Diggle) take on the boomerang-wielding villain Captain Boomerang, while Team Arrow helps Team Flash confront the metahuman Roy Bivolo. A crossover between the two series was announced in July 2014, after Barry Allen had been introduced in Arrows second season ahead of the debut of The Flash. By the following September, the scripts for 
Title: Flash vs. Arrow
Content:  gets his eyes on Thea? There are moments like that, that you just can’t do in a normal episode....It's just so much fun." In October 2014, the title of The Flash episode was announced as "Flash vs. Arrow". Kreisberg revealed that the writers' rooms for both shows were merged while working on the scripts for the episodes, similar to "throwing two casts together". Both episodes were written as self-contained, with Kriesberg explaining, "[W]e were very conscious of not everybody who was watching Flash was watching Arrow and vice versa, so we wanted to make sure they both were self-contained episodes".
Title: Flash vs. Arrow
Content:  The "Flash vs. Arrow" crossover episodes were shown at an exclusive fan screening at the Crest Theatre in Westwood, Los Angeles on November 22, 2014. Afterwards, Amell, Gustin, David Ramsey, Emily Bett Rickards, Danielle Panabaker and Carlos Valdes were joined by executive producers Berlanti, Kreisberg and Guggenheim for a Q&A panel. The first part of the crossover, The Flash, was broadcast on December 2, 2014, followed by part two on Arrow on December 3, both on The CW. The crossover was simulcast with the US broadcast in Canada on CTV. It first aired in the United Kingdom on Channel 5 on December 16 and December 18, 2014. In Australia, the episodes aired back-to-back on January 28, 2015, on Fox8, after Nine Network, which broadcasts Arrow, allowed Fox8 to air the Arrow episode of the crossover.
Title: Flash vs. Arrow
Content:  episode had been written, with filming occurring shortly before the end of the month and into October 2014. The crossover sees every main cast member of each series who appear at least in their own series, with additional actors and characters also reprising their Arrowverse roles. The episodes received positive reviews, with critics calling the crossover fun, and everything fans could have wanted and more. Critics praised the producers for bringing the two series together while keeping the episodes as self-contained adventures, and highlighted the fight scene between the Flash and Arrow as living up to the hype. Ratings for the episodes were exceptional, with both being the highest watched episodes of each show since their respective series premieres. A subsequent crossover occurred the following year titled "Heroes Join Forces".
Title: Arrow (TV series)
Content:  In July 2013, it was announced that Berlanti and Kreisberg, along with Nutter and Geoff Johns, would be creating a television series, The Flash, based on the character of the same name, with an origin story for Barry Allen. The character, played by actor Grant Gustin, was set to appear in three episodes of season two of Arrow, with the final one acting as a backdoor pilot for the new series. However, it was announced in November 2013 that the backdoor pilot would not be happening, with a traditional pilot being made instead. In January 2015, The CW president Mark Pedowitz announced the intention to do a Flash and Arrow crossover every season, and The CW 
Title: Arrow (season 2)
Content:  Neal), whilst continuing to struggle to survive alongside allies Slade and Sara, and the archer Shado (Celina Jade). The origins of his feud with Slade is revealed. The series was renewed for its second season on February 11, 2013. The second season received critical acclaim, and averaged 3.28 million viewers each week. The season would go on to be nominated for fifteen nominations in various categories. This season introduces characters from The Flash, which was being developed as a potential spin-off at the time. The season was released on DVD and Blu-ray on September 16, 2014. The series was renewed for a third season on February 12, 2014.
Title: The Flash (season 1)
Content:  On July 30, 2013, it was announced that Arrow co-creators Greg Berlanti and Andrew Kreisberg, Arrow pilot director David Nutter, and DC Comics CCO Geoff Johns would develop a television series based on the Flash for The CW, and it would detail Barry Allen's origin. Kreisberg revealed after the announcement that Allen would first appear as a recurring character on Arrow in three episodes of season two—all written by Berlanti, Kreisberg and Johns—and the last of the episodes would act as a backdoor pilot for the new show. Kreisberg added that Allen would be a forensic scientist and the introduction of his superpowers, 
Title: The Flash (season 1)
Content:  On July 18, 2014, Kreisberg stated that a crossover event would occur in the eighth episode of the first season of The Flash and the third season of Arrow, respectively. In early January 2015, The CW president Mark Pedowitz announced the intention to do a Flash/Arrow crossover every season, after the success of the first one.

Question: when did the flash first appear on arrow?
Answer: second season.