The shape of the mark, though, is still being sorted out.
One thing that Barry will be responsible for is that Oliver will finally get his mask. During tonight's episode, we just saw Barry ask Oliver why he used makeup instead of a mask to conceal his face while in disguise as Arrow. Oliver's response--that he would need a perfectly-conformed mask, otherwise his vision would be hindered--elicited a surprising response from "The Scientist," as the show called Barry last week:
Basically, he said, "Oh, here's how to do that." Problem solved. And, by way of a proof-of-concept, he left a mask behind for Oliver when he left at the end of the episode. Not a bad day's work for Barry, who's so notoriously late for everything that it seems to have saved his life in the final moments of the episode when the particle accelerator he was so keen to see went kablooey.
And for Oliver? It's one more step toward becoming "Green Arrow"--something that's likely to happen this season, I should think. This episode was notable in that you had Thea calling him The Hood and Barry calling him The Vigilante while his own people called him Arrow (and instructed Barry to do the same). That kind of lack of a focused message, should be helped out by the mask.
And as far as secret identities--it should make quick transformations back to a CEO of a major corporation much more seamless, certainly.
What's arguably the most interesting thing about Barry being the one who brought the idea of the mask to Oliver is this: in the comics, the whole reason Barry called himself The Flash (at least in the Silver Age; I can't remember if this carried over to the Flash: Rebirth soft reboot or the slightly harder reboot of the New 52) was that he grew up reading the stories of Jay Garrick, the Flash of Earth-2 who lived as a comic book character in the "main" DC Universe.
The Flash of Earth-2, by the way, got a shout-out from Geoff Johns, who joked (right? I mean, we have to assume it was a joke) that John Wesley Shipp, who played The Flash in the 1990s live-action series, would be the Flash of Earth-2 as far as Grant Gustin's pilot was concerned.
Anyway, that notion of Barry Allen as a fanboy--somebody who grew up fascinated with superheroes and who has been chasing similar phenomena for most of his adult life before settling in as the #1 fan of a costumed vigilante--could certainly inform why he would be the one to bring the idea of a mask into the equation. Because when you're a fan of superhero comics and somebody is doing this, you have to wonder why they wouldn't have a mask, right?
Also something that was kind of glanced over--Barry Allen's quest to find the truth about his mother has seemingly, based on his own dialogue in "The Scientist," led him to more places than just Starling City at more times than just this once. Where else might he have been tracking strange phenomena--and how long until we find out that some of the characters of the DC Universe are not only quietly active in the world of Arrow, but have connections on some level to Barry Allen?
That might explain Stephen Amell's cryptic Justice League tease from earlier in the season...!