Per Amell's phrasing, the flashback likely takes place in the season finale, although the star did not specify, saying only that he had gone 21 episodes (of a 22-episode season) without a flashback prior to this one.
21 episodes into Season 6 and I still haven’t shot a flashback. So this picture... this picture feels really good! pic.twitter.com/UCx3cQ7q88— Stephen Amell (@StephenAmell) March 29, 2018
Donnell, who played Oliver Queen's best friend Tommy Merlyn, was a fan-favorite series regular in the show's first season before he died in the finale. The character has appeared in flashbacks a handful of times since, and most recently appeared in "Crisis on Earth-X," the four-show crossover event that brought together the casts of Arrow, The Flash, Supergirl, and DC's Legends of Tomorrow.
In the context of "Crisis on Earth-X," Donnell played the Earth-X doppelganger of Tommy, who dressed as the villain Prometheus and hailed from a planet where Nazis won the second World War.
Donnell is a series regular on Chicago Med, which has limited his ability to come back to Arrow, but he remains close with the cast and has popped back up to Vancouver whenever possible to shoot flashbacks and other sequences. In "Invasion!," when Arrow celebrated its hundredth episode amid last year's crossover with a number of cameos from the show's past, Donnell's absence was explained away by a wink-and-a-nod line of dialogue that he was stuck in Chicago.
Tommy is not the only relic of the show's earliest days to be making a comeback in the tail end of season 6: on last night's episode, it was revealed that Ricardo Diaz has taken over the Vertigo trade and is keen to reintroduce that designer drug, which appeared heavily in the show's first two seasons and less so in the years since, to the streets of Star City.
Tommy could, one guesses, be a Vertigo-inspired hallucination, although Amell's comments about a flashback seem to make that less likely.
Arrow airs on Thursday nights at 9 p.m. ET/PT on The CW, following episodes of Supernatural.