Obviously the Once and LOST universes are to be similar. The show share writers, something that is clear to see based on how the format of the episodes work . Flashbacks and reveals and little nods can be seen throughout the series’ from the start. Not to mention both shows are basically “Parent Issues: The TV Show”.
Once Upon a Time makes use of Apollo Bars, Oceanic Airlines, AND Ajira Airways, so obviously they give LOST little nods here and there.
But if you follow Once: Wonderland, you may be able to pick up on something that explains the connection between the two even better.
SPOILERS AHEAD FOR LOST, ONCE UPON A TIME, AND ONCE UPON A TIME IN WONDERLAND: