The Earth-90 version of Arrow's John Diggle was revealed to be a Green Lantern in Monday's "Elseworlds" episode of Arrow. The revelation was offhandedly mentioned by the Earth-90 Barry Allen/Flash (John Wesley Shipp) when talking to several people including Diggle, who is played by David Ramsey. (Earth-90 is the reality of Shipp's The Flash show from the 1990s).
“My name is Barry Allen. Hey John where’s your ring? Guess things are different here," Shipp's Flash said.
Before the episode, Ramsey had tweeted a teaser:
BEWARE..— David Ramsey (@david_ramsey) December 11, 2018
In hindsight, it is clear he is referring to the the Green Lantern oath: "In brightest day, in blackest night, No evil shall escape my sight Let those who worship evil's might, Beware my power...Green Lantern's light!"
The Earth-90 Ramsey did not appear in the episode - and neither did his costume. He could still appear in Tuesday's "Elseworlds" conclusion in Supergirl.