What that billboard does is proclaim that FlashForward and Lost are taking place in the same universe. It’s a universe alternate to ours in which there is no Oceanic Airlines. But it’s a universe in which all the strange phenomena of Lost – including Oceanic Airlines and an island and people that can jump through time – exist or existed.
Oceanic Airlines in FlashForward is in effect a portal that connects both ways between the people and events of Lost and the people and events of FlashForward. Since FlashForward takes place in the present and six months into future, and Lost takes place a little and much further back in the past, that can work.
The people part of the portal are especially significant to consider. Jack in Lost could end up in some episode of FlashForward – not just the actor Matthew Fox but the character Dr. Jack Shephard. Mark Benford from FlashForward could appear in Lost.
In some cases, though, it could get a little complicated. Olivia Benford in FlashForward is played by Sonya Walger, who also plays Penny in Lost. So the actress would pretty much always have to be Olivia in FlashForward, and Penny in Lost, lest the viewers get totally confused and crazy. If Simon, played by Dominic Monaghan in FlashForward (we haven’t really met him yet on the series), were to show up in Lost, viewers would have a hard time understanding why “Simon” looked just like the late Charlie Pace.
Fortunately, Robert J. Sawyer, the author of the FlashForward novel upon which the series is based, doesn’t look much like James Sawyer from Lost, so we would be ok on that score.
But whichever ways these two series go, we could be in for some fascinating crossovers, or, who knows, maybe none at all.
See also FlashForward Debuts
Listen to 40-minute interview with Robert J. Sawyer
review of Lost Season 5 finale, with links to reviews of earlier episodes
6-min podcast episode about Oceanic